Polar recommends resetting the Polar F6 watch if the unit is frozen. Resetting the watch may return your watch to normal. If resetting doesn't solve your problem, have the watch's battery inspected by a professional or replaced. A low battery can be at fault.
Push and hold all of the buttons on your Polar F6 except the "Light" button.
Release the buttons when the Polar F6's displays all available digits.
Push "OK." This is the button on the right side of the watch. The watch will reset.