The first thing, turn off power to the unit to prevent damage to the compressor. Check to see if fan turns by reaching a small stick in to see if it rotates freely, if not the fan will need to be replaced. If it does rotate freely, and just recently turned off, there is slight possibility of overheating of fan motor and the dual capacitor needs to be checked and replaced. Another possibility is check to see if wiring has burned in two or a wire has fallen off connection. If you do not know how long it has been stopped, just changing the capacitor typically does not fix the problem because the motor can rapidly heat up and cause winding damage that is not repairable. In this case you will need a motor replaced. When you get it replaced make sure the tech installs a new capacitor with the new motor.
Hope this helps