You guys talked about ice. Maybe ice you could not see? Don't know what the temperature was. Did it rain the night before? Airplane parked outside and the temp got below freezing? Could have been trapped water inside the elevator assembly, frozen, and acted like another counter weight. Either way, do not fly until assessed.

You could also have a a crack in your horizontal stab structure. Don't mess around with this, don't fly it until you've a got a 100% diagnosis and fix on it.