take off the back cover.
towards the bottom of each side, there is a piece of metal about 3-4 inches long with a notch/slot cut in it. I believe what happened was that the piece's notch slipped off of what it was sitting on. - I know this description isn't clear as I did this 3 years ago; but compare one side to the other side to see what the notch was latched onto.
you push/slide it back onto the latch - may require another set of hands to help work the latch as you push. I struggled with it by myself. after you're done, you will think how simple it was.