Dammit, I knew I should have made bookmarks to the threads I subscribed to.  This really blows.

Stopped getting notifications a few days ago.  Signed on just now to check the at least 30 topics I was following and they're all gone, every single one of them.  I'm the only one using this PC, and I know I didn't accidentally delete them.

Has this happened to anyone else?

No, but I constantly don't have topics with new replies marked as such in my control panel.
There are things between heaven and hell...

subscriptions are automatically pruned after xx days if no replies for over xx days. i think this happened in ur case?

To every single one of them?  No way.  Several were current threads.

Here is what happened. I got several bounced mails every day. People subscribe to topics and change mail, box full, no such user here etc. Because of that i'm using hack to clear up bounced mails. That's maybe what happened to your account. I'm sorry.