If it got wet, you might just cut out the damaged portion and replace with a new piece. If its just come loose, you could try rescrewing with drywall screws into the ceiling joists. I would mark the joists with a chalk line after using a stud finder/magnet to find them. Then use drywall screws with your drill just below the surface and put a bit of joint compound to hide the heads. There are lots of videos on YouTube to watch to identify your best solution.... https://www.youtube.com/results?search_query=how+to+fix+a+sagging+drywall+ceiling
If it is coming loose then you will have to screw it back up with drywall screws. If you paint it again be sure to use a good ceiling paint. You can buy paint which has a color in it and it will turn white when it dries. This makes it easier to see where you have;painted. Putty over the screw nails before painting and prime the areas.
https://www.youtube.com/watch?v=CMfg3rdwnOs
If it got wet, you might just cut out the damaged portion and replace with a new piece. If its just come loose, you could try rescrewing with drywall screws into the ceiling joists. I would mark the joists with a chalk line after using a stud finder/magnet to find them. Then use drywall screws with your drill just below the surface and put a bit of joint compound to hide the heads. There are lots of videos on YouTube to watch to identify your best solution.... https://www.youtube.com/results?search_query=how+to+fix+a+sagging+drywall+ceiling
https://www.youtube.com/watch?v=jhfz2HNuL7s&t=90s
If it is coming loose then you will have to screw it back up with drywall screws. If you paint it again be sure to use a good ceiling paint. You can buy paint which has a color in it and it will turn white when it dries. This makes it easier to see where you have;painted. Putty over the screw nails before painting and prime the areas.