It sounds like your mail files are corrupted. First of all create a backup of your Thunderbird profile folder. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile

Then try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup done prior to the corruption.

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

thank you for that I have just deleted all the emails affected, as I have them on another computer I have forwarded them to my main computer and they now are showing ok. thanks for your help!!

This only applies to older emails that I am trying to go back and view. I tried to rebuild the profile, but in windows 7 I do not get a repair button under properties. I have not had any antivirus software on my computer until after trying to rebuild the profile. Then I tried to install Hitman Pro, but that did not do anything. This has happened in the past and I had not had enough time to try and fix the problem.