pc wont start all lights are on

You have to be a bit more specific.
Have you just built the system or, has it been running for a bit.
And has stopped powering up.

If a new build check you have connected the 24pin atx block from your power supply and the eight or four pin power phase connections from the psu to the motherboard.

The four or eight pin socket is located near the cpu socket and must be connected to allow the board to power up properly.
Make sure you have connected all of the case switches to the header of the motherboard for the power switch PS, and reset switch RS, and any power leads PL, and hard drive leds HL

Last of all check the back of your Psu to make sure if it has a rocker switch it is selected to normal operation of the Psu and not Isolating power to the Psu. Often missed by first time builders. or forgotten about.

If the system is old then you most likely have a problem with the existing Psu.