Well, you're monitor going to sleep means one of two things. Either your monitor is going faulty, or more likely your system is having an issue.
If you have another desktop or even a laptop with a vga port you could easily test your monitor. If you get video then your monitor is fine. At this point you have to figure out what is causing a lack of video to be sent to the monitor.
Do you have an on-board video card(attached to the motherboard) or an add-on card? You can tell this by looking at where you plug in, if your using on-board video it will typically be plugged in vertically, if you're using an add-on card it will typically be plugged in horizontally.
If you have an add-on card then if you feel comfortable, you could remove that and try using the on board video. If it starts working then your video card probably went bad. Get back with this information and we'll see what we can do for you.