That's not as uncommon as you might think. Check where the wall and counter join, and make sure the mounting screws are screwed into the holes in the wall. They have probably come loose, and allowed the backside of the counter to drop. You can raise up the back of the counter again, and reattach it using the screws in the holes, and it would probably be a good idea to add some more screws to reduce the chance of it happening again.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.