First: Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled".

If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

If this is not the issue, then please post your Troubleshooting Information from Thunderbird: Thunderbird menu: Help: Troubleshooting Information Click the "Copy text to clipboard" button. Paste (Ctrl-V), the text into your response here.

Hi Thanks for the suggestion - tried restarting with dsabled add-ons but no change I've attached the trouble shooting log

 Application Basics

   Name: Thunderbird
   Version: 45.6.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20161222195647
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account2:
     INCOMING: account2, , (pop3) mail.btinternet.com:995, SSL, passwordCleartext
     OUTGOING: , mail.btinternet.com:465, SSL, passwordCleartext, true

   account3:
     INCOMING: account3, , (pop3) mail.btinternet.com:995, SSL, passwordCleartext
     OUTGOING: , mail.btinternet.com:465, SSL, passwordCleartext, true

   account4:
     INCOMING: account4, , (pop3) mail.btinternet.com:995, SSL, passwordCleartext
     OUTGOING: , mail.btinternet.com:465, SSL, passwordCleartext, true

   account5:
     INCOMING: account5, , (pop3) mail.styleprint.biz:110, plain, passwordCleartext
     OUTGOING: , mail.btinternet.com:465, SSL, passwordCleartext, true

   account6:
     INCOMING: account6, , (pop3) mail.bikerbuyz.co.uk:110, plain, passwordCleartext
     OUTGOING: , mail.btinternet.com:465, SSL, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) mail.barhammotorcycleclub.co.uk:110, plain, passwordCleartext
     OUTGOING: , mail.btinternet.com:465, SSL, passwordCleartext, true

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-70b8d507-3e4b-4f7a-8663-a33d92160405 (05/04/2016)
   http://crash-stats.mozilla.com/report/index/bp-acad6138-bce5-4bde-a60d-029192160328 (28/03/2016)

 Extensions
   Lightning, 4.7.6, false, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 45.6.0
     font.internaluseonly.changed: false
     font.name.monospace.el: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 17
     gfx.crash-guard.glcontext.appVersion: 45.6.0
     gfx.crash-guard.glcontext.deviceID: 0x0a16
     gfx.crash-guard.glcontext.driverVersion: 10.18.15.4256
     gfx.crash-guard.glcontext.gfx.driver-init.direct3d11-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-d3d11: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-warp: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-try-d3d11: true
     gfx.crash-guard.status.glcontext: 2
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: cff03d52-5bd8-4e49-8a8d-269042a2a2e
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1483014455
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     security.disable_button.openCertManager: false
     security.disable_button.openDeviceManager: false
     security.OCSP.enabled: 0

 Graphics

     Adapter Description: Intel(R) HD Graphics Family
     Vendor ID: 0x8086
     Device ID: 0x0a16
     Adapter RAM: Unknown
     Adapter Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 10.18.15.4256
     Driver Date: 7-17-2015
     Direct2D Enabled: false
     DirectWrite Enabled: false (10.0.10586.633)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: false
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.12
     4.12

     NSS
     3.21.3 Basic ECC
     3.21.3 Basic ECC

     NSS Util
     3.21.3
     3.21.3

     NSS SSL
     3.21.3 Basic ECC
     3.21.3 Basic ECC

     NSS S/MIME
     3.21.3 Basic ECC
     3.21.3 Basic ECC

The next thing to test:

Restart the operating system in Safe Mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP

In Windows Safe Mode with Networking, see if Thunderbird still does this.

Unfortunately still experiencing the same problem

Next thing I suggest is to Create a new profile in Thunderbird for testing and see if the same behavior happens in the new profile.

Hi Powered the machine up again to try the new profile and hey everything works again as it should, so going to quit while I'm ahead! Thanks for the help - I'll hold the new profile in the back pocket if it fails again so for now we'll class it as solved ! Thanks again