I was in your exact position a week ago, thought I had killed my nav unit - but now it's working great, thanks to the tips here.

I found my MkIII was very picky about how the disk was inserted - I had to gently push it in and, if it did not grab the disk, try moving it to the left, right, up and down until the mechanism would pull it in. I forced a disk in once and had a heck of a time getting it back out - had to hold the eject key down for a LONG time (2 minutes or so).

The disk MUST be burned in Mode 2 as mentioned, otherwise you will get a message that it is starting the SW update and then nothing. The good news is that if you burn a proper disk the unit will read it and come back to life even though you think you've killed it.