There are multiple reasons this may happen.  Please contact support with the following information appropriate for your issue:

• Did you receive an error?  If so, what is it?  (A screenshot would be very helpful)
• Did the entire program shut down and restart?
• What were the last few steps you took or clicks you made before the program shut down?
• Are you receiving any errors trying to log back in?  If so, what is it?
• Did this happen to all users on all stations, or just one station?

Please have this information handy when you contact support.