Resetting iWatch 2 - forgot passcode

I'm trying to reset my iWatch and pair it to a new phone. It's an old iWatch 2 that has been neglected for 5 years. I tried the online instructions. The first step is to hold down side button until power off screen shows up while on charger. I have held it for 15 minutes - the power off screen never shows up. Any suggestions on what to do next?