GPS fault / speed showing zero even when moving

Just a thought, but do you have a magnetic speed sensor? If so, the 820 will use that in preference to GPS speed. Could be a problem with the sensor eg magnet alignment or battery. 

Share a FIT file with us. But i suppose a master reset (or a full format) will help.

Do not format your Edge 820. You will lose map and other data that cannot be replaced by downloading from Garmin Express. However, I too am curious about seeing the file from this ride.

If he makes a full backup before the formatting , there is no problem (otherwise only an unnecessary logo file will be lost, and with high speed internet, the map file will be downloaded relatively quickly) . Sometimes the file system became corrupted (due to removing the Edge from computer without usb dismounting) and only the full format helps. Its a common issue (just lots of people dont know how to use their Garmin right with computer...)

Thankyou both. I will share a FIT file - but I need to wait for the problem to happen again. I will hopefully do a long ride on Friday morning, so hopefully I'll be able to share something after that (I tend delete rides from my Garmin once uploaded). I won't format the device - it doesn't seem like a file-system type problem to me. I'll repost once I've got something...

You can download the original FIT file from Garmin Connect: Save Original (under "gear wheel" icon on Activities page , desktop Connect). So You dont need to wait to happen it again. 

Hello. Well, I want for a 100-mile cycle ride on Friday, everything was fine - the problem didn''t happen. However, today I was out for a shorter spin, and it happened four times. (All exactly the same, speed showing zero, but map screen still updating, infact the furn-by-turn directions were still ok, even on auto-pause. I stopped, restarted the Garmin, and continued). 

Then: At the end of the ride, I was unable to sync my Garmin - just wouldn't update, even though connected. So, I've had to connect the Garmin to a computer to copy the file direct into Strava. Strava link here:https://www.strava.com/activities/2492056490

You can see, after 53.9km (just South of Longcliffe, turning left onto Manystones Lane), the data cuts out, route straightlines.

Ref the first reply here,  I DO have a speed/cadence sensor, and although I use this for cadence recording, I doint (and never have) had a magnet on my wheel, so I do not use it for speed. Perhaps it could have got a false-positive reading, which may have impacted it?? I think I will try adding a magnet to see if that improves things...

Funny, when I try to add the FIT file here, I'm getting an error message "Error occured, please try again...!"

try to send me by private message (or rename it to .zip or .rar, probably the .fit is not supported file type) . btw, i checked your strava activity and the gps part is faulty in your 820 (it's typical issue , i have seen it many times) . A master reset will help , 100% sure.

I've started getting this exact same problem recently on my 820. Start the ride, display goes off due to battery saver mode and then some minutes later the screen will come back and the ride will be paused with the speed showing as zero. Try to stop and restart the ride and nothing happens. Every other function seems to be working fine, cadence, direction, maps, etc. Just the recording is not happening and speed is zero. The only way to get around this is to turn the device off and on. It's only started happening recently, maybe after the last firmware upgrade?