Your question was quite vague, but its probably because it is going into Hibernate mode. (if you have Windows 98 or later i believe) It should go to standby after 20 minutes and hibernate after 60, by default. With standby, you can just press a key or move the mouse, and it'll come back. With hibernate, you will have to press the power button agaain. It should come up with a little white bar saying "return to windows." If that all sounds foreign, then you probably have an actual problem, and I don't know what that is!!