I noticed that when I selected emails in my Trash folder it no longer showed me the contents.

That's typically a sign for mail file corruption. What's been causing the corruption I don't know, but my guess is you're storing large amounts of messages in Trash. That's a bad idea.

I right clicked the folder and applied the "repair folder". After doing so all my emails in the Trash folder have disappeared.

Your messages were already gone before you did the repair. The repair just made it obvious.

These emails are very important.

And that's probably the reason why you store them in Trash.

How to I get them back?

If they were very important you certainly made a backup. So you can easily restore them from the backup.

The emails in Trash appeared fine before I ran Norton Anti-Virus, which found some viruses. I rebooted the computer and this is when the emails in Trash stopped appearing.

There was only a year's worth of emails. Hardly enough to corrupt the file.

So it seems that switching to Thunderbird was a big mistake

Norton routinely delete the entire mail storage files when they automatically repair. So I suggest you look in Nortons logs for the deletion of files names the same as your affected folders.

If you are lucky you might find that they are quarantined and you can get them back. One of the trouble with clicking anything automated is you do not know exactly what that automated response is.