Try a force to mass storage.

Force to Mass Storage:

• Power the device off
  
• Press and hold the lap button while plugging into usb (with or without factory cable)
  
• Wait 10 seconds and device should go to mass storage
  

See if this helps: Force to Mass Storage

Then delete the contents of the Garmin\NewFiles folder as mentioned by aweatherall

You did the "force to mass storage" as mentioned by aweatherall?

Force to Mass Storage:

1. Power the device off
2. Press and hold the lap button while plugging into USB
3. Wait 10 to 15 seconds and device should go to mass storage

https://forums.garmin…

The 820 uses the same reset and force to mass storage options as the 1000.

What type of computer are you connecting this to? Mac/PC, Windows 10...

If your unit is stuck trying to boot, then the following may work...

to force to mass storage mode
Mount the unit to the computer, you may need to use a force mount (see below). Remove any files that maybe in the Garmin\NewFiles folder.

Force to Mass…

You could try this

Force to Mass Storage:

1. Power the device off
2. Press and hold the lap button while plugging into USB
3. Wait 10 to 15 seconds and device should go to mass storage

Will it mount to a computer?

Force to Mass Storage:

• Power the device off
  
• Press and hold the lap button while plugging into USB
  
• Wait 10 seconds and device should go to mass storage
  

Force to Mass Storage -
Turn unit off. Hold the lap/reset button while plugging the unit into the USB. Continue holding the lap/reset for 30 seconds and the unit should go to mass storage.

Try
Force to Mass Storage -
Turn unit off. Hold the lap/reset button while plugging the unit into the USB. Continue holding the lap/reset for 30 seconds and the unit should go to mass storage.

Try
Force to Mass Storage -
Turn unit off. Hold the lap/reset button while plugging the unit into the USB. Continue holding the lap/reset for 30 seconds and the unit should go to mass storage.