unlock a locked up tty session

["Michael Martinell" <mike@dakotasioux.com>]

This has happened several times recently.  I have changed from one tty
session to another, come back to the first, and the session is locked up.
If I was in an application, it just sits there, if I was at the command
line, I cannot type.  I can change back to other tty sessions and continue
to do work.  I generally keep this up until I am down to my last session,
then reboot, which clear up the problem, for awhile.

I have 2 questions:

1. How do I log off of the locked-up session without restarting?
2. Where would I look to see what is causing the problem to begin with?