Car will not start

Just replaced the water pump and head gasket because of overheating issues. Two days ago I noticed the AC plug had not been plugged in after the engine had been taken apart, though this is not connected to the starter I just want to mention the mechanic was a family friend. I am not sure if there may be a poor connection from the starter. The brake pads were replaced yesterday and after driving around to check the pads, I turned the car off at the mailbox and had to push it home because it would not turn over. The battery is maybe six months old, but will not jump start or anything. All interior lights work properly. We have also ruled out the alternator because the lights have not dimmed at all. It was very abrupt which has us thinking it is a connection/ electrical.