My apple watch face has come loose, now what?

You don't mention the Series you have. If it's a Series 2 or 3 Aluminum watch you might be covered under Apple's screen replacement program.....https://support.apple.com/screen-replacement-program-apple-watch-series-2-3

If not it might be partially covered if you have Apple Care + or even under Apple's one year warranty if it is a manufacturing defect. Otherwise, it is at your total expense. This is their service pricing.....https://support.apple.com/watch/repair/service/pricing