Thanks, second step worked, just had to hold down for about 20 seconds.
Just had this happen today. I had a software update that didn't seem to go quite right and I thought the unit got bricked by the failed update. I tried the fist solution but that didn't work, the second one did. I plugged it into the PC and the update seemed to get done properly this time. The unit is now running Software Version 13.20
This happened to me today, and the advice below solved the problem (hooray! and thanks to those who provided the advice). But I think it should be stressed that you have to hold the buttons down for a LONG time - I didn't time how long but it must have been at least 10 seconds. The unit's now back to normal, as far as I can tell, and it's now talking to Garmin Express on my iMac, which it wasn't doing prior to that.
Thanks the master reset worked! My 520 is four years old and has been relatively trouble free. No recent software updates. I had to use the master reset a couple of times, but phew, it's working again.
Fantastic, down button and power button pressed for over 5 seconds. Thanks