So I believe I had some head gasket trouble because after an overheat I began losing power while in gear. Eventually the car would start, but when put in gear, turned itself off because of what sounded like too low of an idle (even when pumping the throttle).

I towed it home, replaced the spark plugs, and she turned on without even barely turning the key. When I tried turning it off, the key did nothing and as I couldn't do anything else I pulled one of the power cables off the battery. Now with the power back on she won't even turn over.

Did I break the starter somehow by pulling the power cord? How would I fix that?
Did the spark plugs fix the power problem?