Alternator short. Now it's not cranking. What could have burned?

Hello, I'm not sure what an accidental short at the back of the alternator is. But, the voltage drop to the starter motor is likely due to some kind of power issue compromising enough power going to the starter motor. Or, could be that the starter motor is drawing too much power and will not turn. There's nothing to be suggested or guessed at here. You'll have to check for amperage draw of the starter motor, check the cabling from the battery to the starter and then check all fuses under the hood in the distribution box. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops