Hi my (unidentified) mouse sander won't start(.) After using it on some wooder slices (it) seemed to back fire and stop.
Back fire? You have some kind of petrol mouse sander?
Without knowing the make model, I think you have an electric, mains powered sander. It make a sudden noise while you were using it and then stopped.
If that's the case, I would suggest the motor has probably burned out. It may have overheated. Had you been using it for a long time?
You likely will just have to buy a new one.