If the device has a powered mount plug it into a computer using the USB port located directly on the body of the device rather than the mount. See Where to Connect the USB Computer Cable on Devices With a Powered Mount. If the device powers on this could indicate a problem with the mount and it may need to be replaced.
Why is This Happening?
An issue where your Garmin device will not turn on, is frozen, has a dead battery, or has a black screen can have several different causes:
Dead Battery
A device will not turn on if it has an empty or "dead" battery
This can be resolved by charging the battery
This is not always as simple as connecting the device to any cable or power source.
The device may have run into a software "bug," "glitch," or another problem if the device "freezes" on a single screen and will not respond to your touch.
Since all electronic devices do a large number of tasks and calculations every second, it is always possible for the device to get stuck at one point or another while in use.
This problem may be solved with a software update.