Bad starter, possible. Bad battery too, possible. System status and checklist: 1. Check the battery. Try to get a jump off a running car. If the Mustang Starts, end of story. 2. If it doesn't start then you can try to replace the starter. Take the old one out by loosening the bolts thus loosening the fan belt. Take off the wires that attach (one should attache from the negative pole of the battery, a "harness" will connect several wire going elsewhere its a "plug" may need a screwdriver to pry the clip ends loose. The starter should slip right out. You can take it to any car part shop and have them take a look see - starters are relatively cheap and if you keep the receipt you can return it. I strongly urge taking the older starter with you, making certain that the bolt holes are the same. If the new starter turns the engine over, end of problems. If not, then you have overheated your engine to death. This will require a mechanic unless you have the skills yourself (doubtful since you asked here). you will need the pistons and cylinders worked on most likely. Overheating can cause cracks in the engine block (not fixable, need a new engine) and other things can go wrong - several minor things or one big thing. In all cases, good luck.

grinding instead of cranking, I would check the flywheel

If the motor does not rotate at all it is most likely a bad bendix in youre starter .In any case you shuold pull the starter and check fly wheel for burs or excessive wear.Also ckeck the gear on the starter for damage or wear.If all that checks out be sure to check all battery connections for wear and be sure battery has proper voltage.

Are you sure your starter isn't hangin on the flywheel? if it is then the starter needs to be shimmed.