Non responsive USB ports Power MAC G4 Tower

Two possibilities exist, and odds are very good the both of them will lead to the same ultimate conclusion that you need a new logic board.

The first possibility is if you have a bluetooth module (internal) for the computer. Then you could use a bluetooth keyboard and mouse.

The second possibility is finding a PCI USB add-in card that would work with whatever version of OS X you're running.

But it sounds like your USB controller died, which is about the only way all the ports would go bad at once. Of course it would be a good idea to test this by just making sure you have a USB mouse plugged in (optical), flip it over, then turn the computer on. If you see a red light at any point, then the hardware is clearly functional at the open firmware level and you have an OS problem. That would be what you want to see, but probably what you won't see. And that system is on the vintage list for sure, quite possibly the obsolete, so your only real hope of fixing it would be to scrounge ebay and the likes. Not really a good idea since you never know what other issues may be present, the person may not have any idea how to handle sensitive electronics, pack them correctly, etc.