Replaced battery from right joycon, and now it won’t work
I removed the battery from my right joycon, and after reinstalling it the lights on the side of the joycon would not light up. The joycon would only work when connected to the switch.
Not entirely sure what this might be but some things to check would be to make sure all cables are plugged in and undamaged and if there's a positive and negative side of the battery to make sure they're the right way around. Hopefully this will help and hopefully someone more experienced than me can give you a better answer
This just happened to me on a thumb stick repair. During 4 tear down and reassembles to get the thumb stick fixed, I went to test and the joy con wasn’t powering on. My aging eyes thought the battery was properly connected but it turned out the north side wasn’t fully clicked into place. Gave it a push, it clicked, and now all is well. Also if you somehow managed to insert the connector upside down it won’t work, the contacts are at the end. Holding the joy con vertically, hot south ground north. Good luck.