2012 sprinter won't turn over. But turn over fine when I short the starter

What is your question?

I stop for fuel it's diesel came back went to start it ignition lights up everything lights up but it will not turn over if I leave the key on and go short the starter it turns over just fine nice and fast all the lights are bright but it won't turn over with the key at all it won't it won't start if even if I crank the starter from underneath