Hello,
It's hard to tell from the photo, but looks like it simply isn't seated all the way and/or snapped into place. It may not snap into place all the way if the bottom DIMM isn't properly seated first.
-Which exact model Mac?
-When it dropped, where are the impact point(s)?
-How much and where is the physical damage to the case?
-Are the levers broken at all? They don't look to be in the photo.
-Have you tried removing both DIMMs and reseating them? And.or/swapping them so the bottom is the top and top is the bottom?
-Are you having trouble with both DIMMs, or just the "top" one? If just the top, does it startup without it?
NOTE: Although it takes a little pressure to properly seat and snap in the DIMMs, be careful not to force them and break/damage the slots or levers.
C