You're going to need to tell more information for us to help you. What have you done recently right before it stopped working. IF you put in a new hard drive is it configured correctly? Are the dip switches set correctly for slave and master? Are all the power cables connected right? All the data cables seated? If it is a new hard drive - is there an operating system loaded on it?
If you're getting a fan to come on, your PSU is providing some power. Are the internal power cables connected on the motherboard?
GROUND yourself before touching anything inside.
Turn all power off as well!
Break things down to the basics.
Disconnect everything except your motherboard, RAM, videocard, and power supply.
turn on computer. Do you get a POST? (Powr on Self Test)
if not, you have a hardware failure or something is not connected or seated right - possibility you also have a bad PSU (power supply) IS it adequate for your video card and motherboard? Do not use a PSU less than 450W for most newer systems. Some high end video cards require their own PSU!!!
Reseat all your RAM and video card.
If you get a post, add your other stuff one by one starting with your primary hard drive. retest...
GROUND yourself at all times.