I recently tried to upgrade the software on my Garmin GPSMap 396 aviation GPS. Unfortunately, the webupdater program crashed halfway through the update, leaving the GPS in an unusable state. Turning the GPS off and back on would give me a splash screen for a couple of seconds, followed by a big red screen that said, ominously, "System Software Missing" and a long beep. In this state, the GPS wouldn't even accept a new update; the update program on my PC wouldn't recognize the GPS as even being present. The GPS was bricked. Uh-oh, I was supposed to go flying today!
After much Googling around, I finally found the answer: power down the unit, and while powering it back on, push and hold the "up arrow" key (i.e., the up direction on the cursor pad). Keep holding that button down continuously while re-starting the update program on your PC. As long as you're holding down the up-arrow key, the splash screen stays up, the USB connects, and the updater program can flash a new version of the system software.
Some other Garmin GPS models can be put into this mode by holding down the power button -- that was the most common advice I found on how to solve this problem -- but the aviation GPS lines including the 396 that I own require you to hold down up-arrow instead.
Incidentally, it turned out the reason for the initial crash was because I was trying to update over a flimsy and very long USB cable attached to a hub. Repeated attempts to update always resulted in a crash. When I finally attached the USB cable directly to my PC instead, the update proceeded normally.