Which type of CPU do you have socket 478 or 775? (478 pins on CPU, 775 pins on board) Be sure there are no bent pins on either and if so perhaps you can fix them by carefully straightening them with very gentle pressure. Avoid to much back and forth as you can break off the pins and that is a sure way to finish it off for sure.
Did you remove the CPU from the heatsink and reinstall it? If so be sure that you lift the retaining arm before installing it and secure it with said arm after installing it.
If you did not remove the CPU from the heatsink before reinstalling then please carefully remove it from the heatsink, clean both and reapply thermal paste before reinstalling. When reinstalling be sure to apply slight pressure downward on 478 CPU while closing the retaining arm as sometimes they will not seat completely. Hopefully your back working but maybe this will help.
When a system does not complete POST it will sometimes provide feedback in the form of beep codes or voice. Some makes of boards allow the beep codes to provide failure points. My ASUS board has the voice post and if you connect headphones to the on board audio out you might find that you can hear what is being said. If you can understand what is being said perhaps you can resolve this with what it says and resolve your problem, if not try more below
Further if your system worked before this and you did not force anything causing damage then you should make sure that you reseat everything that is required for the system to work and remove all that are not. This means that you just need board, CPU, memory, video and power supply. Reseat these items and remove all others, be sure to remove power to extra devices to. Be sure to make sure all these items are firmly seated, as poorly seated CPU, memory and video cards often cause post failures. Also be sure that you have fully connected your power and none of the cables in the connectors have come loose (I have seen a few power supplies with a few pins unseated cause failure to post), also be sure the 4 pin additional power is connected to the board. In this case usually only CPU fans will cause failure to post but you did not say it shuts down which would also be a sign of this. But you might try a new or different CPU fan just to be sure. Well hopefully this has you working.