a. Remove the batteries. Both from the left (will display the serial number as well) and right sides. b. Disconnect the external power source. c. Re-install the batteries. d. Connect the external power source e. Try to reset.
If this does not help, then it would be best to call our Support line at 800-282-4103. The Technician answering the phone will need the serial number of the Yuma 2 unit. This can be found when screen is face down and the battery pack has been removed from the left side.