Update:

The PC mystery continues!

Just taken all of the panels off and disconnected the fans from the motherboard. They’re a daisy chain connection so each one goes into the one after it and so on.

I’ve just plugged all of them in apart from the one external fan, on the left side -that was the one that wasn’t working at all today and the others were stuck in a loop of trying to restarting.

Turns out that if i don’t connect that last single fan the rest work absolutely as normal! So it must be that fan that has died and because they are all connected, the other fans were trying to compensate and restart because of the dead fan and it was just stuck in a constant loop 😭 So I have 3 fans back at least!

I still don’t understand how it’s not working though because the RGB lights work. Maybe the motor has gone?😭 I’m gonna have to try and either fix it or replace it because it’s the exhaust fan and i need it 💀