Then you are probably really *(%^ off right now! And you have probably found that when connected to your computer, the Nuvi does not show as an attached device.
However, I have a solution for you.
The update to your Nuvi failed, and the maps are not on it.
Take your Nuvi, and press on the message (which nearly fills the screen) for about 5-10 seconds. The message will then disappear. At this point you (if you have a password on it) will have to unlock the device.
Then attach it to your computer, it should now appear, and the updates (using garmin express probably) will then continue and complete.
Your brick should now be functional again.