Searching for pages about why Thunderbird suddenly stops displaying any emails leads to lots of pages that suggest that the index files might have become corrupted and suggest either deleting the index files manually so that Thunderbird can rebuild them the next time it opens or using the option inside of Thunderbird itself to do the rebuild. Unfortunately there are situations where rebuilding the indexes doesn't resolve the problem.
So what do you do when it turns out that rebuilding the index doesn't fix the problem? Unfortunately the searches I did only found pages that offered that suggestion and so when this occurred with an email account using IMAP the simplest solution I could find was to install a different email program. As soon as that email program finished synchronising with the server, all of the emails were once more accessible through that new email program even though they couldn't be accessed in Thunderbird.
When the same problem occurred on a different computer where the email account used POP the solution was not so straightforward. With POP the emails are not retained on the server and so gaining access to them is not simply a matter of opening the new email program and waiting for it to load all the emails again. As the most obvious replacement email program (Windows Live Mail) uses EML format for storing the emails while Thunderbird uses MBOX format you can't simply import the emails from one to the other and since Thunderbird doesn't display the emails you can't export them to a format that can be read.
The solution to this is to get a separate program that can convert from MBOX to EML format. Fortunately such programs exist and in fact attempting to use such a program gave me a clue as to why Thunderbird was refusing to display the emails. The conversion program actually gave an error message when attempting to read the MBOX email fines saying "Access Denied". Now computer problems are far easier to resolve when you have an error message than they are when things simply don't work. Had Thunderbird given the "Access Denied" message then resolving the problem would have been far easier and would have avoided all the time wasted trying to fix indexes that weren't broken.
The cause that I discovered for Thunderbird not displaying any emails at all and simply hanging wit nothing happening is that the security settings on some or all of the email folders had been changed so that the current account did not have create or write access. This meant that as soon as the program attempted to save the first downloaded email that the program simply got stuck trying to update a file to save the email when it did not have access to write to that file.
In this particular case the emails had been moved to a separate folder on a different drive. Right clicking on that folder (while Thunderbird is not running), opening the Properties and then going to the Security tab showed that the user account being used had no write access to the folder and the problem was fixed by simply changing the security settings to give the user account write access to the folder and then telling it to apply the same change to all the files and folders contained inside that folder.
So if the emails in one folder disappear then it is most likely a corrupted index as all the other pages about this problem suggest BUT if the emails disappear completely in all the folders in Thunderbird then check the security settings on all the folders that Thunderbird references first as if it is the security settings on a folder causing the problem then fixing that means that you don't need to waste time rebuilding indexes that are not broken.
Of course this isn't the only cause of this particular problem. I have also come across a situation where neither of the above were the cause of the problem. In this instance it eventually became obvious that the cause was on the server end and was something changed by the hosting provider. With this one I was unable to do anything to fix things until moving the hosting where the emails were being stored - at which time things started working again without needing any changes to the email account on the local computer.
This article written by Stephen Chapman, Felgall Pty Ltd.