The very first thing to look for is a build up of dust and other rubbish in the case. You can also look for some wire ties the plastic coated ones sometimes these get used to the determent of the computer but they make the cables look neat.
If there are no obvious sings of a short pickup the case and shake to see if anything rattles around sometimes screws get dropped and can cause the M'Board to stop working. Then if there is nothing obvious remove any plug in Cards and the RAM and reseat them all and see if it them works. Sometimes just lifting something out and re-seating it will make the unit work again.
If this is a new unit and hasn't worked from the beginning the most obvious thing is that the Video Card is incompatible with the M'Board this is something that is quite common with AMD M'Boards generally a different maker with the same chip set will cure the problem or you may need to go to a different chip set video card.
Really without more information it's almost impossible to tell what is happening here particularly if you just get a blank screen with the fans running. A little more information as to what is going on here would help and if this is a new build and things like that.
Col