The heads of nails or dry wall screws used to fasten plaster-board to studs often pop out. This is caused by evaporation of moisture from the studs and settling, which together squeeze the nails out.
To remedy popped nail heads is simple. First drive additional drywall nails about two inches above and below the popped nail head to secure the drywall. Drive the nails, including the popped nail, until a recessed dimple shows around the head, but do not break the paper face. Be sure you hold the plasterboard tight to the stud.
Dust off all loose paint and plaster and fill the dimple with joint compound, applying with a wide putty knife. Let dry, and since patch usually shrinks, re-apply after lightly sanding. When your patch is completely dry, sand with fine sandpaper and paint.