It's a well-known issue on S9k but if it still spinning physically you can try this below...
How is that an S9k issue? this is more like every fan on planet earth's issue, running for months with probably little no cleaning is enough to toast any fan, it really has nothing to do with the S9k model itself, S9k is indeed the worst miner as far as the control board and hashboards are concerned, but it's a matter of luck when dealing with fans, I have some fans that spin for nearly two years at 90% without a problem, some die in a few months, you just got to blame your luck and move on, there isn't really any way around to fix those fans given their cheap prices.
OP, get yourself a new fan/s as Phill said, and I would advise you to actually get an extra one because pretty soon the other one might stop working too, a spare fan and a spare PSU are really important, this is why owning a single miner is probably a bad idea, because buying a spare fan, PSU and a control board for a single miner is not the best thing to do, but if you have a few of them and keeping a single piece of spare of everything makes a lot of sense.
On a side note, I am not sure if this just by chance, but I noticed that setting a fixed fan speed on those fans even at high RPMs of 90%, the fans tend to last longer than those set to auto-mode, maybe the continuous change of the motor speed kills it faster, or maybe that was just my luck.