unfortunately L&P ceilings do that. The nails rust away over time and the laths rot, especially if there has been damp or a leaky roof or pipe, and the ceilings were mostly shaken loose between 1940 and 1945.
It can be triggered by slamming doors, or by walking about or storing stuff in the loft.
Best action would be, as @Octothorpe says, to have an experienced person repair it. An economical way is to have plasterboard screwed up from below, and plaster skimmed. But you also have to work from above to remove the plaster nibs that have fallen down into the gap between the old plaster and the joists or laths because they will prevent the plaster being lifted up. I've done it by hand and with a powerful builders vac.
If you want a cheap bodge, you can put a sheet of ply or other board under the ceiling, propped up with "deadmen" which is a builder's name for props. It will hold it up and prevent it falling on the person below.
Don't pick a plasterer from advertising websites where they pay to be listed, even if it masquerades as a recommendations website.
Ask you local historic buildings or residents association. A specialist can re-attach the original ceiling rather than using plasterboard.
If you are above-average handy, it is possible to DIY, but would take to long to explain here.
If it does fall down, the dirt will be unbelievable, and lime plaster is a sort of thick cement, it will cause damage and possibly injury.
If it falls down, or you have it taken down, cut around the coving and cornice to retain the original features. These were made in a Victorian factory and nailed up by the plasterer separately to plastering the ceiling, so there is no need for them to be destroyed. They are fairly easy to repair if chipped or cracked, by a person familiar with this work.
Only use screws. Do not use nails as the hammer blows may shake loose the fragile plaster.