No map data availble

Your Garmin GPS has an internal memory or an SD-card that contains an "operating system" and "map data".The operating system is like windows or Mac OSX. It contains the menu and everything the GPS displays. The "map data" is a digital roadmap the GPS uses to calculate your route. If the map data was on a SD-card that was inserted in your GPS, it could me that you lost the SD-card. That could be the reason your GPS can't find the map data. Sometimes it happens that when people connect their GPS to a computer to update the "operating system" of the GPS something goes wrong. For example, when you disconnect your GPS from the computer while the update software is still writing data to your device, the connection gets lost and you're left with corrupt files on the GPS. It could also be a defect in the internal memory of the GPS. I suggest you try to update the operating system with a program called "Garmin Express" (just google and easy find it). You need to connect the GPS to your computer with a USB cable. Then you can perform an update. I hope it helps to solve your problem. What you could also try is to soft reset your GPS. To do that, you need to remove the silver or blue frame around the screen of the GPS (it pops off). Behind this border there is a reset button.
If the map data is completely gone, and you don't have a backup there is only one solution: Buy the latest map for your GPS online and download it to your GPS. It's expensive. In that case it's better to buy a new device in a shop. Most new devices contain Lifetime online map updates (model name nuvi -then the number - LM or LMT) (LM= Lifetime Mapupdates T= Traffic information) Then you can update your map data every three months if you want.