USB mouse and keyboard not working

Any advice/thoughts/suggestions appreciated:

All 3 USB ports at the back of my iMac have stopped working. This means I can't use either a keyboard or mouse, so have no way of logging in. I turn my iMac on, it boots up as usual, but when it gets to the login screen, the mouse and keyboard don't work and I get a message about bluetooth set-up. (Both my mouse and keyboard are wired, not bluetooth).

I have tried an SMC reset (by turning the iMac off, unplugging all USB devices and the power cable, leaving the iMac for a few minutes and then plugging the cable back in a turning it on again). This didn't work.

I've also tried a different version of the SMC reset where I hold down the power button whilst all the cables are unplugged and also as I plug the power cable back in. After doing this, the fan turns on at full speed (v loud), but the mouse and keyboard still don't work.

I've also checked my mouse on a different computer and it works fine.

Has anyone got any other suggestions / workarounds?

Note I can't do a PRAM reset because my keyboard doesn't work.

Is it probably that the USB hardwear has failed? If so, is it possible to repair this? It is worth repairing given it is a 5/6 year old iMac (the white plastic one) which is probably on its last legs anyway?

many thanks!

Jez.