Hello and thanks for your question. I look forward to assisting you today with the best answer possible.
What is the model of your Garmin?
1. Turn off the unit.
2. Press and hold down the power button and you will see "Pre Boot Mode" appear on the lower part of the display. Keep holding(60 seconds or more) the power button down until the pre boot mode message disappears.
3. Then wait for the device to initialize.
Tell me how it goes!
Would you like me to remote in your computer and work on the issue?
We need to download an updater and a recovery file and then we need to run the recovery file via the updater.
What is the Operating system of your computer?
Please click this link: http://www.justanswer.com/remotedesktop/join.aspx?Session=9484cbd0-aabb-40ff-a470-4ea6894fde62
Then click "I Agree"Then download and run the Elsinore Screen Connect.
Download the Recovery Application and recovery file for your device, and save them on your Desktop.Drag and drop the .rgn file on top of the Updater.exe Recovery Application. The application will launch.
No problem take your time.
I will give you a guide which will help you restore your unit.
Please click this link to access the guide: http://rickbeardknerd.blogspot.in/2013/01/restore-garmin-nuvi-760.html
Just follow the instructions in this guide.
Keep me posted!