Kitchen ceiling

If the plasterboard has dropped by about ½ inch, filler alone won't fix it. The board likely needs to be re-secured to the ceiling joists with plasterboard screws, then the crack should be taped, filled properly, sanded, and repainted. Also check for any water leaks or loose fixings, as these can cause the board to sag in the first place. If it's genuinely dropped that much, replacing a section of plasterboard may be the best long-term repair.