Soft reset the watch by pressing and holding the middle button on the side until the device forces off (takes about 10-15 seconds of holding) > Then release the button and briefly hold the same button again to power the watch back on.
Once the watch is rebooted, see if the Action button on the upper right side of the watch resumes normal operation to open the activity menu. If you continue to have concerns with the Action button operating on your watch, I recommend following the steps outlined on the Garmin Support page linked below:
