Howszit? Supaman1982 and Welcome to GtoG...
The 1st thing a system will do is try to find the video output. If it cannot find it, the system will just sit there waiting for it. Since it starts sometimes, it could just be a dusty set of memory chips.
Does your system have a video card or is the video coming from the motherboard, (if its a motherboard video system, the video connection will be near the keyboard and mouse PS2 ports). How familiar are you with the inside of your system? Have you ever looked "under the hood"? I ask this because the 1st step you may need to do is re-seat the video card and/or memory chip(s) after cleaning them. Just make sure before you do this, unplug the system from the mains and try to make sure that you haven't built up a static electricity charge. Also examine the CPU heat sink and make sure that it is also not full of dust.
What is the system make & model? If we know that we can better advise you...
Hope this helps....