Hey! I know its a n olkd thread but i have exactly the same Problem. What was the solution? The Link provided(forums.garmin.com/showthread.php) does not work anymore.
Thanks!
The forum was completely changed, so any links will go nowhere. jlgranth might be able to provide some information. There are other threads that describe a fix - https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-230-235/235769/hr-stopped-working-on-235
Unfortunately, I don't remember the exact fix. I would suggest reaching out to product support. In the mean time, you could try holding down the power button until the screen goes blank (takes about 15 - 20 seconds), then release and power it back up.
See the youtube video where steps 1, 2, 3 and 4 fixed my 620 heart monitor.
Note some instructions for replacing the battery say "wait 30 seconds" which may be to allow something to discharge - short circuiting the contacts presumably guarantees it is discharged.
1. Remove the battery from the heart monitor
2. Short circuit the two_battery_contacts_in_the_heart_monitor with a paper clip - NOT the battery itself!
3. Put battery back
4. Pair the monitor with the watch.
It may be useful to reset the watch to DEFAULT settings as well.