Part of the battery door must have broken for the battery to end up inside the aid. Most battery doors are plastic and break easily. I used to keep a supply on hand when I wore CICs because I needed to replace one about every year.
Anyway, without an extra door and the small hand tools that might be required to extricate the battery, I’d take it to any audiologist or HA dispenser for assistance.
P.S. Many battery doors simply snap on and off but don’t try that unless you’re sure how yours is installed.