Could be what BUGIE said,
or... there is a screw at the bottom of the glovebox directly in the middle (in the footwell basically) When I installed my amp i unscrwed a few of these to get the wiring hidden - I didn't screw the 1 in the centre of the glovebox in properly and my glovebox opened like your describing.
Try feeling for this screw and apply pressure to the bit of plastic around it and it should pop back up and the glove box will fully open.
Sorry if that sounds a bit confusing