If it is the internal cooling fan, please do not attempt to use the microwave (other than for quick tests) until it is fixed. You can overheat and destroy the magnetron.
If it's the hood / vent fan, then you can go ahead and sue it, but you should set up a small fan to keep any stovetop steam or smoke out the microwave controls until it's fixed..
Please reply back with your full model number and let us know which fan has failed.