My new Garmin Epix Gen 2 updated to 15.76 and is now locked up at the "Reset default settings?" prompt.

The update has messed up a lot of watches, it stopped the elevation on mine had to reset and restore from backup.

try holding the top left button till it turns off, will take around 15 seconds 

This is usually a sign for a defect „back“ button that is constantly pressed. Common issue on the Epix/Fenix 7 lineup. 

Try cleaning the button under warm water and pressing it often. 

(Could also be the update, but until now never read it)