How to get COPMUTER out of SLEEP MODE?

My computer often goes into sleep mode. This happens so often. It is a Windows XP. Especially if I am away from my computer for more than an hour, it goes into sleep mode, so the screen is all black. Sometimes when I return, it is fine, and turns onto the main screen. But other times, it stays black, but all power is still on. Then, when I punch any numbers or move mouse, nothng happens. So I try pushing the Menu or Auto Select buttons. All that happens is, the text: Computer Going to Sleep Mode, appears, and it goes all black again. So, the only way I can figure out get the compute back on, is to do the emergency shut down. I know this isn't good for our computers, and hate to do it. Plus, then I have to wait so long for it to reboot. Can anybody tell me how to wake the computer up when it goes into sleeping mode? Would be greatly appreciated. Thanks in advance!