The PSU is still good and I'm using it right now in my backup rig so that's not the problem. Also, I have a good surge protector on all my stuff so I doubt that's the problem.
A power system is more than just a power supply. Furthermore, a defective power supply can still spin its fans or boot a computer. To know the condition of all power system components and to know if that supply is really good: can only be answered by numbers from a multimeter. IOW disconnect nothing. Not even one wire. In one minute of labor, take some VDC numbers from six wires between supply and motherboard. Only then will an answer say what is defective ... without speculation.
A surge protector is only as effective as its earth ground. View the manufacturer specs for that 'good' protector. How good? Its protector circuit is typically a circuit that sells in a grocery store for $7. Put some fancy paint and higher price on it. Then most will call it a better protector. Plug-in protectors are not selling surge protection. For if it did, then you can list numbers from its numeric specs. A number for each type of surge it claims to protect from.
Protectors that are too close to appliances and too far from earth ground have a history of making surge damage easier. Superior protection was (should have been) inside that computer.
To say what has failed without spending money shotgunning starts with numbers from the meter. Details provided if you really want an answer. Therefore have already purchased or borrowed a multimeter that costs less than a good hammer. That sells in stores that also sell hammers.