It may be defective - seized bearings (possibly) -jammed up blade (perhaps) - open circuit motor winding - wire fallen off - no power arriving at fan for some other reason.
You do not mention the make & model of your oven.
This is not a very sensible answer - sorry - an appliance engineer may be able to diagnose the problem and resolve it.
In the meantime you will just have to cook without the fan.
Good luck...
Peccavi, June 2010
