those snap in.
option 1: lightly use your palm and give it a small light hit (it might clip in).
if that fails see option 2:
option 2: remove your mud flaps and rear bolts under and in the rear of the wheel well. Gently pull the bumper outwards (they will begin to unsnap) you can feed your hand in there to help push where the seem is outward.
Check tab. Mine once folded over - was lucky to fold it back and put it back int he hole... (hate it when that happens...)
thats not it see option 3:
option 3: congrats! your 4runner is totaled. Call your insurance company and tell them you need someone to come haul it away cause it needs to be crushed....
lol honestly option 1 or 2 usually solves it.
Take pictures if you get to takin the bumper off might be able to help you out.
Good luck