Then I thought I had accidentally switched the Fenix 5S off. Tried to switch it back on and it hasn't properly switched on since. Have plugged into the mains and it says almost zilch battery, hasn't charged either.
Restart and Reset the Garmin Fenix 5. Sometimes, a simple restart or reset can fix charging issues caused by minor software glitches. Begin by restarting the Garmin Fenix 5 by holding down the power button until the device powers off. After a few seconds, press the power button again to turn it back on. The two-pack of this Garmin charging puck:US Amazon: SV6bj.