While unplugged, you could open the case up and double check for any obvious signs of damage. Next, check to make sure that all cards, memory, CPU, etc.... are still firmly seated in their motherboard slots. Also, double check to make sure that all data and power cables are still firmly connected, and that the motherboard is not grounding out. Afterwards, see if it may power up.
If all of the above checks out, then you may need to take it to a computer repair shop to have it checked out if you do not have any spare parts to swap out to test with.
I hope it's something simple.