2004 Pontiac Grand Am engine over heating

Two days ago my sister was driving her car and she drove through a big puddle on the street that was due to the snow melting. When she drove through the puddle the car was covered in water about 3/4 of the way up the tire. The car had stopped moving when she pressed on the gas there was no response so she put it in Reverse and pressed the gas but still no movement she waited a few seconds and applied gas eith more force she finaly got out. She then proceded to drive for about 20 minutes and the " low coolant" light came on she bought coolant and nothing changed the engine over heats after about 5 minutes of driving. What could be wrong with the car?