Sleep mode

After installing Windows XP I have had this issue with not being able to start my monitor if I have been away from my computer for a while. It seems to go into sleep mode which is what it is suppose to do but it doesn’t matter what key I hit or what button I use on my mouse, the monitor remains black. My hardrive however boots up, the light on the start button stops flashing and is solid once I touch a key or my mouse. Can anyone give me any suggestions as to how to fix this? It is becoming quite annoying.

Thank you for your time

Eube