Hopefully, you have a backup of your unit's contents, especially the map. If so, buy another sd card and install. Connect to the computer, start Home, and it should offer the application to be installed on the card. Do so. And copy back your backed up map folder.
If you don't have a backup (why not?) then hopefully the map you did have on the card was an update from the original map with the device. If so, it's still on your computer in zipped format. Use Home to look at Items on my Computer. You should see a map listed. Select it and install on the card.
If you have neither a back up of the map nor the map on your computer, I'm afraid you'll need to purchase another one from TomTom.