Is the power LED solid or flashing? It makes a difference.
If it's flashing, then it's a simple power supply replacement. If it's solid, it's usually a result of some add-in card or RAM has come loose somehow.
My other computer is a Dell Inspiron 530. It's been working fine but when I turned it on this morning it won't start. The indicator light is yellow instead of green but nothing else happens. It's probably a no brainer but if somebody could help me out I would appreciate it.