Toilet model would help.
www.thetford-europe.com click on Service
then service information, then toilets then cassette toilets select your model range select repair instructions. The following is what they don't tell you as they want you to buy a new pump.
Possible
pump failure may be impeller jammed to find out if current is getting to the pump switch on the 12 volt light in the toilet operate the toilet switch observe if the light dims then current is getting to the pump so the impeller may be jammed or pump knackered.
micro switch knackered if its got one.
Now for the download repair instructions one pdf covers all ranges find your range look how to access the pump take it from there. Just remember DO NOT operate pump out of water dry pumps damage easily so ensure you have the pump submerged in water before testing.
Pull pump out turn it up so you see the base and gently with a small screwdriver turn the impeller about six full turns. Replace the pump back in the tank, ensure it has water in it then try the pump.
Most common pumps stick when tank not drained when in storage a long time.
Bob UK
May 2014