Those pictures are quite close up, but there's no farther back shot that would let us really see
where in your trunk that is.
I'm guessing here - you have a car that doesn't have fold-down rear seats (I didn't know they still existed!
), and that's the panel that is the back wall of the trunk, essentially the opposite side of your rear seat back.
(even if I am wrong, and it is a panel suspended underneath the rear deck, or even a side panel, the solution is the same)
We can't tell you specifically "what to do", because every car make and model is different. But we can tell you how to troubleshoot the problem.
It simply requires exploration, and using your brain to understand what went wrong based on what you see in that exploration.
You need to get back there, and look first at whether the back panel is attached to the metal framework of the car, or to the seat back itself. Is that panel designed to cover and hide the metal framework, or is the panel designed to be trapped behind the metal framework?
Then, you need to figure out what the attachment method is. If it is attached to the metal framework of the car, it may use screws (if it did, there may be a protective/decorative cover snapped over the screw head), but chances are it uses some sort of speed connector. There are plastic push-in "snap" style push-ins, plastic push-in "brush" style push-ins, and plastic push-in connectors that use small plastic screws to spread four plastic legs to secure the connector in position. Look for them, if the back panel is secured to the metal framework.
If this type, I'd also suggest removing everything from your trunk first, so that you can actually get in there to see.
If the back panel is attached to the seat back instead, it's probably easiest to simply remove the seat back panel. With most cars, this is a simple affair, there are somewhere between one and three bolts located in the space where the seat bottom and seat back meet. The bolts go through a wire loop or metal strap that is attached to the seat back. You remove those one, two, or three bolts, and then lift up on the seat back (the top is held in place with clips on most cars). It will pop out of the clips, and free. Then you can figure out how it attaches very easily, even bring it inside to work on it.
Usually, only cars with fold-down seat backs have those covers attached to the seat back itself, where cars without folding seats have the panel attached to the back panel.
If the problem is that a screw or connector is missing, if it is not something available at your car parts store, it is something that is available from your local car dealer for less than a buck.