My troy-bilt 5550 started yesterday - won't today

Open hood and look at left side of engine compartment. Find the braid jumper going from engine block to fender. Grab hold and wiggle. It may be loose from corrosion at the fender end. If possible tighten. If not replace with a new heavy jumper from block to body of car. This a major design flaw since battery charging current and starter current is forced through this skimpy wire and connection. I replaced battery and alternator twice before discovering this defect.

Sounds like the shift cable has broken

Either its a bad new battery or tour altenator isn't charging it. Jumpstart it.

those vehicles are common for starters did two today alone when it wont start try to tap it with a hammer it on the back side of the engine under the intake if it starts replace starter

Could quite possibly be you have a bad starter. I would have it tested. Could also be a bad battery. If, when jumping it, you used too high of a voltage, could have shorted out a wire somewhere

If it won't crank over, it's not a fuel issue, but a starting circuit issue. The gas may be stale, but if tank was full all that time, I don't think that would be a problem.
Pull off the small wire on the starter solenoid, and see if it has power when key is held in crank position. If power there, the starter or solenoid or the battery connections from battery to starter need to be checked or tested. If no power to solenoid when key is in cranking position, the ignition switch may be bad or the neutral safety switch (or the clutch start switch if a manual) or the starter relay needs to be checked.
If you get it cranking, and still no start, look down the throttle body bore and watch your injector spray pattern while the engine is cranking-you should see a fine, cone spray pattern coming from the bottom of the injector. If it just drips and no cone pattern, or no gas coming out, injectors need cleaning (probably from stale gas). Good luck.

The battery ran out of power because the alternator is not charging it.
Have the alternator output tested and take a look at the fuses.

If it won't crank, the battery could have failed or you could have a faulty connection between the battery and starter.

That would make sense, because when he jumped your car if he left it hooked up for awhile. Then he charged your battery up, so it started the next day. Then you drove to his shop and back it ran the battery down. It's easy enough to test and change alternators you could take it to autozone or O'Reillys if there is one close they can test your alternator. If you have more questions about this or if you would like to try and change it yourself just reply to this post and I will walk you through it.

This is going to sound strange but, have someone get over to where your starter is. While you try and start it have them tap on it slightly with a small hammer.
I t could be the brushes are hanging up.
Otherwise, I would pull the starter and have an auto center test it.
Please let me know what you find.