Engine question

I had an oil change done last Monday and there was a knocking/grinding sound by my front passenger side, my husband was told that it was a loose bearing. My car stalled out while running last week driving to work and we restarted it and it turned off again. AAA came and said that the battery was low and the alternator was reading negative voltage. A mechanic came and took off the serpentine belt and disconnected the alternator and battery while we were waiting for a new alternator to replace it. During this time we were able to reconnect the battery and start the car and move it to more level ground to be worked on. Keep in mind, the belt and the alternator were disconnected. The new alternator was put in yesterday, but now the car wont' start. It makes a loud noise like it wants to start but won't. The mechanic said that the engine in carbon locked. Prior to all of this there were no check engine lights and no leaks, or overheating. Could it be the engine or something as simple as needing a new battery?