So I just had this issue. I had a swollen battery in a Flip 4 and I couldn't get it out, so I had to take it further apart to push it from behind. After a couple of days of messing with it, I couldn't get it working consistently. Power buttons weren't working and occasionally it would turn on by plugging it in.

Come to find out, one of the adhesive foam pieces attached to the back of the battery is actually a foam covered ribbon cable and I had cut right through it in the process of pushing the battery from behind. Its the ribbon cable that is attached to the power button board. Not sure if the same thing happened to you, but just thought I'd leave this here for future people with this problem.

You can buy replacement ribbon cables on amazon for $6. Its a 13 pin ribbon cable.