There are many reasons why a computer won't power-on. A dead computer may seem to be dead when the power cords are loose. Check the cords first before proceeding.
Now that your computer is certified dead. Here are some of the reasons why it is dead:
1. damaged Power supply Unit(PSU)
2. loose cables(inside the case)
3. damaged processor
In any case, unless your ribbon cables are loose, a dead computer will need new hardware. read this article for a more elaborate fix.