Hello And thanks for your question!
Press and hold the power button for 20 seconds and the unit should reset itself.
Give this a try and let me know how it goes.
I hope this solves your issue, If so please click the green accept button, if not im still here to assist you,
To perform a master reset on nuvi 30, 40, and 50 series devices:
Note: A master reset will erase all waypoints or favorites and will restore your unit to the default factory settings.
The device will now reset. Follow the on-screen prompts and then place your device in an area with a clear view of the sky until it acquires satellites. This could take up to 15 minutes.