Cookerhood Extractor fan not working - Help please

Its the extractor fan over the electric cooker .

It slides in and out and always automatically came on when pulled out .

It must work by motion sensor or something similar -

The bulbs are not blown , as sometimes we get lucky and it does light up and start extracting the fumes.

What could be wrong .?

Not a motion sensor, more likely a microswitch, this may be wearing out or mebe a loose wire. The hood will have to be disassembled and examined. Isolate the electrical supply to it before.(at Fused outlet or mcb).
If you're not an experianced diy'er I'd call an electrician.

I sympathesise with your predicament at this time of year!