Hi @Maymay
Thank fully damage not to great yet I would use no liquid nails to glue behind the cornice be very care full to just place the glue on the wall and press the cornice onto glue.
Press the cornice on to the glue it wont stick right awy so let the wall transfer some glue and let it sit 10 minutes then re press cornice to the wall this time it should hold like using contact cement. Just to be sure drill a screw or two under the cornice to hold it weight in place by morning it will be fixed.
Remove screws.
Clean up any loose paint egdes and peeling and use no more gaps into the cornice and wall joins , use caulking gun and push excess in with finger if necessary till smooth
A clean wet WHITE cloth will let you wipe away any excess and you may not have to repaint the wall egdes.
Always use white clothes for white area clean ups so you know they are clean. Dark clothes can mark white areas.
Hope its fixed soon
as that cornice has moved ?? might be wise to check the rest of the room for problems
