Is this a pop or imap mail account?

If imap - It is entirely possible the Trash folder was already empty on the server. After restarting the account fully synchronised with server and Trash can only display what is on the server. Or you selected the TRash and forced a synch with server, so it updated the view. Not sure what server you use, but many auto empty Trash after X number of days. Some delete TRash every 3 days others 7 days and gmail do it every 30 days.

What server are you using ?

Do you normally exit Thunderbird on a regular basis or just minimise - sleep etc? If no, then perhaps you have a setting to empty Trash on exit, so it only occurs on an infrequent basis. It just so happened to occur this time because you restarted Thunderbird.

This is a POP account. I use thunderbird to access my mail from Breezeline. I had over 20,000 emails that I have kept in my trash going all the way back to 2008 and with all of the updates I have done through the years, they have always stayed in the trash, that is, until I upgraded to 102.1. After doing this I opened my trash folder and they were all gone. I just someone can tell me where they went and how I can retrieve them.