Rozy, it would help if you could post some pictures so we can see what you see.
Off the top of my head, the most common reasons for the screen or back of a phone coming off are either from physical damage such as being dropped, or the battery is swelling and forcing the phone open. Follow the Back Cover Removal guide to check on the battery to see if it's swollen.
[guide|138922|Pocophone F1 Back Cover Removal - iFixit Repair Guide|new_window=true]
If it's swollen the black outer layer will be pillowy and soft, whereas a normal battery it should be solid and tight up against the battery. Again, pictures would let us figure out if it's a problem or not, but a failed battery could easily cause both of the symptoms you're seeing.
[guide|21499|Adding images to an existing question - iFixit Repair Guide|new_window=true]