It has be popped out, there are springs above the drywall restraining it.
Try prying it at one point with a putty knife.
Turn off its breaker & be careful that a bunch of insulation or mouse droppings don't come out with it. I'd recommend a helper.
If you hang it from the wires then a wire may come out of the Wirenut connector and it won't be obvious which Wirenut it goes back into.