Had my 235 for a few years now. Used it a week ago in a 5 hour hike. Since then it will not hold a charge. Twice I have charged it to 100% and the next day it is completely drained, even though I did not use it for any activities.
Probably a bug, try reset - for some users it works, but for others it doesn't.
Similar thread: Forerunner 235 Battery is draining since software update.
Yup, for some it just doesn't, no other fix posted yet for such case.
Maybe battery needs replacing (but kinda strange to lose so much capacity suddenly), if it's a bug, then replacing won't help.
Other option was a paid exchange for a refurbished watch, but I don't know if it's still proposed for this model, and could be cheaper or similar to just buy a used one privately.