if it is still in warranty, send it to be repaired, because the fix voids the warranty.
If it isn't under warranty, take the casing off, see how the fan is connected. Normally it is just 2 or 3 pin but it is often glued down and is sometimes a soldered fan, sooooooo, get a new fan they are pretty much all 120mm fans in the power suplies. Put the cables in, you may have to cut the old ones and the new ones, strip both back, twist and solder. Make sure the wires are long enough aswell, so don't go cutting them right at the base

. When they are in, screw the fan in, put the cover on, fire it up and see if the new fan is up and running
should note aswell that sometimes the 2 pin fans have smaller, closer together pins than your average fan, so even if you can take it out, you may still have to get your soldering iron out, even if it is just to swap plugs over
If you can't remember the direction of the fan when you took it off, the side with the wires coming from the middle and the 4-points holding the fan itself to the outside almost always, and i mean 99% of the time, blows, so uncovered=suck, 4-point cover=blow
Oh, and if you don't have a soldering iron, you can twist the wires together, cover each seperately with insulation tape or thermal shrink wrap or something that will cover the wires and stay on, then tape them together quite far either end of bare wire to take any stress or strain but make absolutely 100% sure there is no bare wire so the fan can't short on the other fan wire or on the side of the case or worse, on a power supply component, then you could short the whole thing out. Once you have them twisted and taped, that should stay forever because the wire isn't ever moving