My fiberglass pool has lost about 2 feet of water since I put the cover on it. What could be the problem?

Check to see if there is a leak in it. You can do this by taking the cover of the pool and marking the water level, then take a bucket of water mark the level on it. place the bucket next to the pool and leave it there for a few days. if the level in the pool drops the same as the level in the bucket then the reduction of water is the result of evaporation. however if the level in the pool drops more then the level in the bucket you are loosing water somewhere.