I made a few photos (pictured: FP3 with old camera module) of the area concerned so people might be able to grasp better what we are talking about here.
What got (and should not have gotten) disconnected here is probably the invisible opposite end of the connector that you can see in the first photo marked by a thin lined triangle. That opposite end should be on the “underside” of the metal that bears the camera icon (just where the tiny mini triangle to the top right of the camera icon is).
On an FP3+, I also found that clicking the connector back onto/into the camera module didn’t work on the first try. I then pushed the (loose) camera module towards the upper end of the case (core module) and the connector clicked into it then.
Eventually, a tip I think can be repeated every now and then: To just test if a module works or works again after you screwed open the FP3 or FP3+, it usually suffices to firmly press together display and core module – you must hear the plastic clips clicking again though. If the plastic clips are properly reconnected, you will usually be able to boot and fully use the FP3 even without the 13 screws back in. However, if the test is successful, don’t forget to still screw the 13 screws back in. 