Thank you Will!
This worked for me in My Fenix 5 today :)
Just did it on my 935 today - solution still relevant
Hopefully you've resolved now, but in case it's of use to others I also fixed mine with a simple switch off and on. Using Connect I could see that it had stopped logging after a I ran the battery flat mid run a week ago. First time I've lost logging, and first time I've run battery flat mid activity, so maybe a connection.
Perfect, soft reset worked great for just this fault