I had the same problem with my Garmin Nuvi 765. When it was powered on, it used to come up to "loading Maps" and hung there or went back to power on status repeatedly.
The 8 Secs resetting procedure mentioned in the user guide also did not help either. Finally the dealer gave me a wonderful tip called "Factory resetting or Master Resetting". ie: Switch off the unit. Keep touching the right bottom cornor of the Unit screen. Under this condition, power on and continue to touch the right bottom, till master reset is completed.
Hope this would solve your problem for your unit also. Good Luck.