Please share more information, such as any error messages.

Thunderbird will not load. Only shows error message - Your Thunderbird profile cannot be loaded. It may be missing or inaccessible

This can happen if the profiles.ini file (which points to correct profile) is missing), or if profile has been renamed. Assuming the profile exists, try this (assuming you're using Windows): - click windows key and the 'r' key together. That brings up a small window - in that window, enter thunderbird.exe -p that starts profile manager - click 'create profile' , then click 'new', then enter a short name (e.g., PROFILE) and then click the browse button to select and activate your profile. The place to look will be c:\users\yourID\appdata\roaming\thunderbird\profiles

I've reinstalled Thunderbird but lost all my saved messages and address book. I don't have files including appdata\roaming\thunderbird\profiles

Reinstalling can sometimes destroy what you're trying to salvage. Did you find any folder named 'appdata' anywhere? If not, then you may have missed it, although it is possible that your profile was destroyed earlier. Anyway, since you have reinstalled, click help>moretroubleshootinginformation, scroll down to 'profiles', click 'about:profiles' and that will open a pane showing all found profiles. If there is more than one, then one of them may be your old one with addressbook and accounts and messages.

You say 'click help' - where? on this website or on thunderbird? I can't find 'help' anywhere - there's Get Help but that doesn't bring up moretroubleshootinginformation. My computer may have been corrupted as I've just lost the info on a page in Excel, and I've had trouble updating my website

Ok, I assumed you had Thunderbird running. Start thunderbird, ignore the prompt to create an email account and click the 'home' tab. then, right-click and on the next small menu, tick 'menu bar'. Once that is done, click 'help' on menu bar and then click moretroubleshooting and follow the above steps. Sorry for the confusion.

Home tab? Nothing apparent with that name on Thunderbird. What appears to be the Home tab only gives the options to Open in a new tab or window. Thunderbird doesn't seem to be able to find my Plusnet account which may be the main problem. My gmail account is OK.

Hold the ALT key down, click Help > Click More Troubleshooting Info

In that new tab, click about:profiles

Is "default-release" the profile currently in use? If you can't tell, please post a screen shot.