problem with virtual terminals

[Paul Kaplan]

When I boot I have 6 virtual terminals.  When X is running it is accessible at 
CTRL-ALT-F7.  In the past, I have been able to start a 2nd X session from VT2 
that becomes accessible at CTRL-ALT-F9, and then switch back and forth 
between the two sessions from the keyboard.  Suddenly, when I try to switch 
back to CTRL-ALT-F7, I'm met with a blank screen, none of the virtual 
terminals are accessible, and I can't get back to the 2nd X session.  My only 
recourse seems to be to reboot.

Any ideas what the problem is or how to trouble shoot.
TIA
Paul