2001 Chevrolet Cavalier

2001 Chevy Cavalier, 2.2, automatic, 77,000. Motor became very loud, oil is fine, but car overheated. Motor spins but will not start, although after waiting a week I tried it again and it did turn over for a short time. Mechanic checked plugs and said there is no compression in one plug (coolant backup? I didn't understand what he meant by that). He says it could be a gasket, or worse but won't know until he opens it up. Could this be a timing belt(chain) issue? Frankly, I can't afford to begin a $500 job and have it turn into a New Motor job. Any suggestions what I can ask him next or what you may think the problem may be?
March 12, 2007.

What it sounds like he trying to say is that one piston
has coolant fluid in the cylinder, that would come from a head gasket or a broken rod. Did he say what the compressions ran?

Mar 12, 2007.