To diagnose problems with Thunderbird, try one of the following:

• 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.
• 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.

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Please also add the result of these troubleshooting steps to your post

• Do you use anti-virus and firewall software? What is the version?
• Who is the email provider?
• Who is the internet provider?
• What is the exact error message?
• What steps did you take right before this happened?

I finally got it to work in safe mode. Deleted all the old emails and emptied trash. Seems to be working o.k. now

Good Did you compact your folders? Keep_it_working

re :I finally got it to work in safe mode.

This could mean that one of your addons is causing a conflict. You would need to disable each addon in turn to see which one it is.

Maybe an addon needs to be updated.

If you peanutsmum said

I finally got it to work in safe mode. Deleted all the old emails and emptied trash. Seems to be working o.k. now

Did you in fact make any permanent changes to your list of addons? And do you have the calendar addon installed?