The USB fan has probably drawn too much power from the USB circuit and therefore caused part of the circuit to fail.
My previous post was not meant as a solution as such, just a workaround if needed. £180 to replace the motherboard is a reasonable price considering the amount of labour involved in stripping the laptop down and rebuilding it. I don't know the exact age of this laptop but I would contact the retailer, in writing, and look for some compensation towards the cost of the repair. Even if the laptop is out of warranty there is still an "expected life" of a product during which it should not fail.