The back came off my phone

Hello! You can get new replacement pre-cut adhesive for it from eBay, or an easier option is T-7000 adhesive or 2MM Tesa tape, though you don't get water resistance back with the last 2. There should be guides online on how to reattach it. I would make sure your battery is not bulging as they shouldn't come off on their own unless it was a previously (badly) repaired phone.