The first thing you want to do is reset the CMOS, it is different on every motherboard but there is usually a jumper near the battery that is labled CMOS or clear CMOS (it will be in the manual and on the motherboard makers website). First unplug your computer from the wall, then pull the jumper off of the pins it is on and put it on the other ones (there are usually 3 pins, with the jumper on 1 and 2 for example, just pull it off 1 and 2 and put it on 2 and 30 for a few minutes, then put it back, plug in the comp and see if it will start up.
If that doesn't help then you will have to reseat whatever you took out of the computer (CPU and RAM or whatever else you took out). If it still doesn't go try reseating everything, even the stuff you didn't touch (ie video card).
As a last resort you can take out everything except the CPU, video card and one stick of RAM, if it still won't go try clearing the CMOS again and think about ordering a new CPU.
Post back and let us know how it goes or ask any questions you want. Telling us what brand and model of motherboard as well as other system components could help too.