Montana 700i not powering on

Montana 700i, stopped reading garmin purchased maps off sd card after last update. No help from a support ticket. Another user suggested to install an older software version. After doing this it wont turn on or connect to the PC. Garmin comes up on screen for a second then red light blinks and it turns off again. Does the same when plugged into the PC. Is there any way to undo this or force it to connect to the PC. I've tried the hold down power button until red light then double tap brings up a green light then shuts down anyway.