This depends on if you want to fix it yourself or not. If you rather do it yourself and save money you need to remove the fan so you can test and see if it's the fan itself such as the motor that has gone bad or if it's what powers the fan. Then you can call to get part.
It is probably a bad motor. I would call a repair man or if you feel like a do-it-yourselfer call a repair parts place. You can usually order the replacement parts from a place and do it yourself pretty easily and cheaply too.