You can do a quick cover-up, but it’s best to firmly set the cornice back in place.
Gather your supplies
• Cornice cement
• Plasterboard screws
You’ll also need
Paint scraper; bucket; drill; lint-free rag or sponge; fine sandpaper; painting equipment; sealer undercoat; acrylic paint to match ceiling colour
Use paint scraper to remove any loose paint or plaster along crack.
Mix cornice cement in bucket to form a paste. Use scraper to fill gap between cornice and ceiling with cornice cement.
Use drill to screw plasterboard screws up through cornice into ceiling so cement oozes out of crack. Remove excess cement with scraper and use to fill screw heads.
Leave cornice cement for a few minutes to start setting, then wipe along crack with damp lint-free rag or sponge for a smooth finish.
When cement has fully dried, sand lightly if required, then apply sealer undercoat. Let dry and follow with two top coats using acrylic paint to match ceiling colour.