try removing the addon, restarting Thunderbird and reinstalling. Sounds to me like it might be in some way corrupt.

I tried this and still not working. Agree something may have gotten corrupted. I installed TB w/ Lightning on another computer and seems to work ok. Can I transfer some files to this other computer so that I can get my old calendar data back?

Can you post a screenshot of your Add-on Manager with the entry for Lightning visible? http://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

See if this helps. https://support.mozilla.org/en-US/questions/1067957

Here is the screen shot. AOM2 is what shows after I hit "more" in AOM. When I hit restart, TB38 starts again but still no lightning. I also went to question 1067957 shown above. I can remove disabled lightning from TB, but when I reinstall it will not start. Still shows disabled.

Try to update to Thunderbird to version 38.2.0 first. That should also update Lightning to version 4.0.2.

I installed the updates, unfortunately, still no lightning. I get the same screens on AOM, except it shows version 4.0.2 for lightning. NOTE: The email portion of TB is working fine. I installed TB w/ lightning on another computer and it seems to be working ok. If we can't fix lightning on the first computer, is there a way to transfer the old calendar data to the new computer?

The guy here suggested this worked for him.

Revert to 31.7.0 (https://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/31.7.0/win32/en-US/), reinstall Lightning 3.3 from the AddOns menu, then reinstall TB 38

I would be most interested if it was two for two.

It almost worked. I installed TB 31.7.0 and then Lightning 3.3.3 And there was lightning with all the old data. Thank You. When I tried updating to TB 38.2, the update downloaded ok, but when it went to applying update, the little circle kept spinning. After 10 minutes, I closed the window and TB. When I restarted TB it showed the update complete. When I went to AOM, it showed lightning 3.3.3 was incompatable. I removed 3.3.3 and tried to install 4.0.1.2 This resulted in the old problem of lightning disabled and not being able to enable again. I went back to TB 31.7 and after some trouble, was able to get lightning 3.3.3 working again. As I am writing this, I see I should have tried to install lightning 4.0.2 with TB 38.2. But right now I am afraid to leave what is working and try the update again.

I also had problems getting Lightning 4.0.2 enabled after the update to TB 38.2.0. What finally worked for me was downloading the candidate for a specific language (en-US in my case) from https://ftp.mozilla.org/pub/mozilla.org/calendar/lightning/candidates/4.0.2-candidates/build1/win32/ . If you update to TB 38.2.0 and run into the same Lightning problem again, you may try the candidate build for your language. Basically the difference is that the candidate doesn't have all localization's built-in. Create a backup of your Thunderbird profile just in case before trying, so you can always revert to a working version if needed.

I finally tried updating to TB 38.2 again (from TB 31.7) After update AOM showed Lightning 3.3.3 was installed but incompatible. I went to the AOM "Get Add-Ons" page and installed Lightning 4.0.2. It then said restart and 4.0.2 would install on restart. But it didn't, just said it was disabled. I downloaded the file Chris1 mentioned, went to AOM and installed 4.0.2 from the downloaded file. This time it worked great.

Again thanks for your help, together you solved the problem. I guess I can't click "solved the problem " for both of you. But I appreciate all the help. Joel