can't happen... The blades don't come off, they BREAK off...
However, you can often repair the motor.
The fan motor's end cap is swedged on. There are chisel notches on the body of the motor. Use a cold chisel and knock the swedged metal back to allow the end plate to be removed. You might need a file to complete the de-swedging operation.
What you will usually find is that the motor brushes have gotten stuck into their holders. The heat from the burning contacts further afix the brushes.
Clean the armature and brush assembly. Use a good grade electrical grease on the brush holder and reassemble. Reswedge the end plate.
I've had more success than failure putting them back together like that.