Q: What is wrong with my car?

I was driving just around the corner from my house, everything seemed to be going fine with my car until all of the sudden the car seemed to be losing power and the check engine light came on flashing. I pulled over and turned off the car. I tried to start it again and immediately when I did the entire car started violently shaking. I turned it off and came back to the car later, when I tried to start it again it would turn over like it's about to start, but it wouldn't start. I checked the coolant in my car and I had filled it up about a two weeks prior, but there was only half left. I brought it to a mechanic and he said that the engine was seized. That the car must have been running hot for days and that I need a new engine. I wasn't sure if that was right because I read online that if the engine is seized it will not turn over at all and mine did. So I brought it to another mechanic and they said that the problem with it is the head gasket. Does that sound like what it could be?

My car has 168000 miles.
My car has an automatic transmission.