First check it has not been jammed by something. Sometimes a sticker will drop off or curl up and jam the fan.
If the fan is not simply stuck and it is within the warranty period, return it for repair/replacement.
Exactly which GTS250 do you have? Get onto the Newegg site and see if they have one there, then link to it so we can see.
As a temporary fix, get a case fan of suitable size and attach it using zip (or cable) ties but do not keep running the card without cooling, you will damage it and that may void the warranty.
There are plenty of good aftermarket coolers around, but they are not all available worldwide...So, where do you live?