Someone on Groundspeak recently posted about problems they had with a firmware upgrade on their Garmin GPSMAP 60CSx. If you don't know what I'm talking about, you might want read this firmware primer -- it's like getting a free upgrade for your GPS.
But what do you do when firmware goes bad? If your unit locks up and doesn't work following an upgrade attempt, before contacting support, try doing a hard reset. You can usually find the combination of keys that does this by searching online for your model name plus the word "reset."
Often times, locking up is not the problem. There is just a bug in the firmware that annoys you and you'd rather go back to the older version. The Groundspeak post mentioned above turned me onto a great resource for this -- a web page of archived GPS firmware and software. How cool is that?
There are a couple of other things I'd like to mention. First, if you're a Garmin owner using WebUpdater, download the most recent version which fixes a problem causing WebUpdater to not recognize when the update process is complete. Second, find a GPS message forum that covers your model. See what other people are saying about a new upgrade before downloading it. Or be adventurous, be one of the first to try it, and then post about any problems the firmware may have.