What’s happened to my coral beauty?

[i cant think]

I suspect something in the water you added for your water change was significantly different from your tank - like the pH, alkalinity or salinity and it shocked the fish. They don't normally recover from laying on their side unless it was something like that.

I’d suspect most likely salinity as many LFSs overdose or underdose salt and don’t always get it good straight away over here.