A sleeping computer

Generally just hitting the spacebar will wake it back up, but some systems aren't very cooperative. If you don't want it to go to sleep at all, you need to find out where that setting is. Generally it will either be in your BIOS or configured in your Control Panels under power management. You should look through the options you have and decide what power management (if any) you want.

If you run into trouble tracking the settings down, let us know some more details. I'll be glad to try to help further.

Good Luck!