Same thing happened to me...Apple Watch just fell apart without incident, Apple refuses to help because it's 1.5 years after purchase and they help just up till 1 year after purchase
A simple google search shows the issue of Apple's Rear Watch Casing falling off after a couple of years is widespread; please see just the first three apple discussion threads that came up when doing a google search:
Back of Watch fell off - Apple Community
Back came off my watch - Apple Community
Apple Watch cover falling off - Apple Community
Yet after a disappointing and frustrating conversation with an Apple rep via text on Apple Support App, another frustrating phone call with an Apple Rep, and finally someone who understood English (took till my 3rd tier of contact at Apple for someone to understand the case isn't cracked, it just separated into two halves without incident) the 3rd apple rep told me Apple will not help with this issue because the device is "277 days past warranty". In other words, it's 1 year, 9 months old and Apple won't stand by their Watch product enough to fix an issue they know about already (see above links) that normal use, sweat and heat, causes the interior adhesive to deteriorate and the device to fall apart.
They're knowingly charging $1k for these Watches they know will split in half less than 2 years from purchase. I'm not even an athlete, I don't run marathons. I go on neighborhood walks and it just fell in half.
Mine is the stainless steel apple watch series 7.
I glued the case back on and now the heartbeat sensor doesn't work so it locks every 10 seconds despite me wearing it and it can't sense any vitals. I believe the tiny ribbon cable holding the front case to the rear became detached in the process and now it's glued back shut with that cable loose, damaged, or unplugged.
I can't complain enough how sub satisfactory Apple's Support and Customer Service is. Too big to fail vibes. Not standing behind their products. Apple is a big disappointment on the customer service front; at least it has been in my experience.