I have included this because if you have had a leaking shower or something similar then you may well have an unsightly water mark on the ceiling. To address this is quite simple you need to block the mark before you repaint it, the best way to do this is applying PVA directly to the affected area, the ceiling needs to be dry, before you start and it does not hurt to apply a few coats, but let each coat dry first. Then give the ceiling a couple of coats of emulsion, which should do the trick, however really bad stains my take several more coats with PVA and then emulsion but they will disappear.
You may have a little more damage than just a watermark, it could of caused a crack in the ceiling, or if it is covered in some form of wallpaper, it may well of caused this to lift, and in extreme cases, the ceiling and any coving, may of even collapsed under the weight of the water.
The wallpaper, will re-stick with normal paste, but the ceiling needs to be completely dry before you do this, I would just let the paper hang there while its drying if you can, then glue it back in place, if your lucky the paper will sit as it did beforehand, however if it leaves a gap because of shrinkage, just try adding some fine filler to the area and gently blend in when dry with fine sandpaper. Once it has, PVA the area and repaint.
If the ceiling has collapsed then to be fair this is a much bigger job that will require the ceiling to be re-boarded in at least the affected area, and it would be better to look at re-plastering the whole ceiling, this size of job would be covered under your building insurance and if this is to big a job for you, just call us for a quote.