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
Automatic Answer
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