Make/model or system spec including psu?
Did this PC ever run before?
What was the last thing you changed/did before the symtoms started?
As a quick fix attempt:
- power off and pull *all* USB cables from the tower. (Oh this is a tower, right? not a laptop?)
- remove keyboard and mouse if not done already.
- power up and let it boot as far as it will.
- power down. Install keyboard and mouse. boot up.
If it works, plug in USB cables one at a time. All should be OK.