Recently my Garmin suddenly goes into autoreboot while I was driving, and it was unable to load the map even if it was powered down and restart.
The problem was believed to be caused by too many Favourites and custom routes created on the device. After performing a master reset the device was able to boot normally, except that all the Favourites created were wiped out, so make sure you backup all your custom data on the device every now and then.
Had my 765t refuse to take an extra route, (done in Mapsource) until I deleted others. Only had 3 small routes on the GPS so it appears that the onboard memory may be too small to store more than a couple.