we have seen a few of these calls lately. if you have the computer set to automatically log out after a certain amount of time (let's say 2 minutes) and the clerk hits the credit card button. if they get caught up talking and don't do anything and that time elapses, it will log them out. THE CREDIT CARD SCREEN IS ON TOP! they do not realize it and think that retail pro is frozen up. simply move the credit card screen and log back in.
(watch out for this also if they are not setup in workstation preferences to close the credit card window automatically after the card is authorized)