The rebuild process did the trick.
I had already tried using the menu command to empty the trash.
Search now functions again. I'm realizing that it may not have
worked because the search I tried was searching the trash, along
with two other mailboxes.
When this happened, there weren't so many messages in the trash
- not even close to 1GB. Just the most recent two months. But,
a couple of months ago, I had been pruning the database, and had
deleted gobs of old mail. Given that, it's interesting that
this incident didn't happen then. It was probably the fullest
the trash mailbox had ever been.
Thanks for your help!