I do not know why it happened, but 65535 is 1111 1111 1111 1111 in binary transcriptions. In other words it is the maximal value of a short 16 bit unsigned integer. So certainly something went wrong. Resetting the watch to factory defaults is probably the only way to fix it. If it does not help, contact the Product Support through their website at https://support.garmin.com
why this happens? 
