how can i fix bottom oven?

Disconnect from supply, open door and remove as for cleaning (don't worry if you don't know how). Unscrew back panel and lift panel away, behind should be the fan and element, chances are that the element has failed and you may see a white/grey residue. Unscrew element and carefully pull through terminals and wiring, disconnect and check that element has resistance of about 25-30 Ohms, and no continuity to earth. If it's failed then replace and refit element - when it's all back together turn it on and double check that the fan turns when on.

Other reason could be the thermostat, turn on and you should hear a click as it activates, if it doesn't then that'll be a tad more expensive to repair. Good luck.

BFT
April 2008