Please explain in detail how you did carry over your existing profile to the new Thunderbird installation.

I didn't do anything to the profile. I restored W10, found Thunderbird wasn't working - some error message about a missing file which I don't remember. I then reinstalled Thunderbird hoping/expecting this to solve the problem - as it had with the other applications that needed fixing after the reinstallation. Instead Thunderbird shows the behaviour described that makes it unusable. Since the mail is downloaded and can be seen with effort; but none of the pop ups such as 'new message' work, I think the back end is working it's the front end user interface that isn't.

I restored W10

I don't have Win10 and I don't know what it means to your existing data when it is 'restored'. So I don't think I can help much further with this.

What always baffles me is why people don't seem to bother to create a backup of their Thunderbird profile (and any other important data) prior to potentially disruptive actions.

In MS parlance 'system restore' means 'restore the system files from a backup while leaving the user files and applications alone'. Would that I had had the opportunity to take a last backup of the TB profile or anything else before my computer crashed last week and was rendered unusable with (I presume) corrupted system files!

For what it's worth to others, this is what I finally did to sort out the mess: 1. Extract my TB 'profile' into a new directory tree from my last backup; 2. Start TB pointing at the new directory tree. It now works; but, of course, the mail etc is only up to the last backup date; 3. Stop TB; 4. Replace the mail folders in the new directory tree by those in the old directory (which I suspect to be OK even if TB can't process them properly); 5. Start TB - seems to be working as it should be! 6. Delete the old directory tree, which presumably is somehow partially corrupt.

Therefore I conclude that some corruption had occurred in TB processing between the data files (which seem to be OK) and the user interface (which seems to be OK); whereby the user interface could not process the data files properly, symptoms of which included an inability to create new windows, like the 'write' window, and the inability to update the 'Home' pane with messages. I note that none of the other applications I rebuilt behaved in this bizarre way.

In this case I think just reinstalling Thunderbird might actually help.

I think all your flailing replacing the profile folder will have simply synchronized the copy of lightning in your profile with the copy of Thunderbird.

Each lightning only works with one Thunderbird and if they get out of sync version wise the symptoms you have listed, no message list, dialog not opening etc are the usual result.

Partly/Largely in desperation I uninstalled and installed TB several times and the situation was not changed. The symptoms I had certainly sound like the ones you describe for 'lightning'; but what is 'lightning'? It's the first time I heard about what was likely the cause of several wasted days for me.

Lightning is the calendar add-on bundled with Thunderbird. So you do not waste time in the future, restart with add-ons disabled from the help menu (Or hold shift while you start Thunderbird) is the first line of attack with problems.

Safe mode strips all not default themes, gets rid of add-ons and modifies the use of hardware acceleration, as well as some other things I do not understand, this has a high likelihood of success in fixing "Thunderbird" issues. Anti virus scanning of mail and Anti virus firewalls are the high probability fix for "I can not get mail" if the account worked yesterday. Far higher than the provider having some sort of issue. Actually higher that account settings. But everyone goes straight to changing account settings.

It is worth noting that most people that experience this issue see it over and over with every update. but they are like 1 in 10,000. While the mechanism that causes it is understood, fixing it has proved elusive. It may well be that there are multiple causes of the failure and the developers are not yet aware of them all.

However the following has been standard advice for people with your issue and it has helped quite a few.

              -------ooO0Ooo-------

Did you just update to XX.X. If so, this will probably fix your problem.

1. On the toolbar then 
2. Select extensions and remove the Lightning (calendar) addon
3. Restart Thunderbird
4. Reinstall Lightning/calendar addon - if you need the functionality

Please tell us, does that help?

Note

1. If you can't get to Add-ons, then start Thunderbird in safe mode by holding the shift key while starting Thunderbird.
2. If you still can't get to Add-ons, simply reinstall Thunderbird from http://getthunderbird.com/

Thank you, I will store this information for future reference (and hope I never need it). I did attempt a TB restart with add-ons disabled but evidently what was wrong prevented this. I did not try starting TB with add-ons disabled (shift key) and, fortunately for me, it's too late to see if that would make any difference now. As I said, I reinstalled TB at least twice and this didn't solve my problem.

For what it's worth to others, this is what I finally did to sort out the mess:

Can you mark your post as the 'Solution' for this thread please? https://support.mozilla.org/en-US/questions/1129290#answer-894153 Thank you.