My first generation apple watch back came loose

Hi Jenny

If the issue is not covered under warranty or any other Apple policy - be sure to ask, with reference to potential battery swelling issues - then your Apple Watch may instead be eligible for replacement via chargeable, out-of-warranty service or via AppleCare+, if previously purchased.

Contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your Apple Watch to be inspected and to establish your available options:

Apple Watch Informationscenter för service – Apple-support

I'm afraid that no-one here can tell you what Apple will decide in your specific case. You are not talking to Apple here - this is a user-based technical support community.

In the meantime, note that Apple advises that you should not continue using a damaged Apple Watch.

This happened to me this week, too. I was shocked, as I have also taken good care of my watch and it has worked fine all along. I was also bummed, as I had purchased an Apple Care policy, but it expired last summer. I took the watch and my receipt to the local Apple store and they said Apple knows about this issue and is extending the warranty on first generation Apple watches to 3 years for this particular problem. They took back my old watch (they gave me back my watch band and charger) and sent it to Apple, and I received an email yesterday that a replacement watch is being shipped to me. So hurry up and contact Apple or the Apple store as soon as possible to make sure you get in before the 3 year deadline on this. Good luck.