Ummm, it would have been best if you started with that piece of info.
Just to clarify, turning the key on and starting the engine aren't the same thing. When you turn the key on all you are doing is turning on the electrical system. "Starting" the engine means turning the key all the way forward and engaging the starter.
Pull the plugs and try to crank the engine over with the starter. If it still won't go put a socket on the front of the crank pulley and try to turn it(with the plugs still out). If it still won't turn over then you can stop right there. Its seized and you may as well go look for another motor.