Are you talking about the auxiliary fan at the condensor outside, or the blower fan in the a/c unit under the dash? If you are talking about the blower fan, it sounds like you have a total power loss such as a blown fuse, OR you have a slipping belt which is causing the compressor speed sensor to shut down the system. Does it try to work after restarting the motor? That's how you reset the system after the compressor belt slip sensor shuts it down.

Good luck,