Data appears to be missing

What data?

All of the folders for the email accounts, the sub-folders for inbox, sent mail, and so forth and the contents of those sub-folders.

You'd need to provide a little more detail about your problem. Please explain in detail

• what you were doing
• what happened

You may also provide a screenshot. http://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

In case you don't have a backup of your profile create one. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

I was running Thunderbird -45.0a1.en-US.win64.installer.exe I upgraded to Thunderbird -46.0a1.en-US.win64.installer.exe After the upgrade the following screen appeared. Reverting to the 45 version and all of my email accounts and email information was restored.

V46.0a1 is a very early development build, and it can always break things. When you run development software you should know exactly what you're doing, and always have a backup of your Thunderbird profile.

The current release version is 38.5.1. https://www.mozilla.org/en-US/thunderbird/

Chris, the build that you pointed out was a 32 bit version. I was running the 64 bit version and it was running quite well. I was pointing out a speed bump with the last release. It isn't the first time that it has happened. I thought I would share my observations.

I reverted to version 45 and it is working well.

the build that you pointed out was a 32 bit version.

Of course, because you didn't mention you're running a 64 bit version before. You neither mentioned you're running a development build in your OP, which would have been useful information.

I was running the 64 bit version and it was running quite well.

Testing development versions is fine. But as said before, you should know what you're doing in the first place. There is an official 64 bit version for Linux, but AFAIK there's none for Windows. See http://kb.mozillazine.org/64_bit_builds

I was pointing out a speed bump with the last release.

Again, v46.0a1 is a very early development build, and not a release version. It can break any time, and usually related questions are not raised in the support forum. You may raise a bug in Bugzilla though, but that should then really be a bit more substantial than what you provided in your OP here. Or ask in a developer forum.

nightly builds have been broken for several days. iirc they are close to having a fix

p.s. http://forums.mozillazine.org/viewforum.php?f=29 is a much better source for getting information and asking questions about daily builds