Many happy Barnes & Noble Nook Color users were horrified yesterday to discover that their beloved Nook Color, during an over-the-air (OTA) firmware upgrade, had reset itself, possibly erasing all data and leaving users with a factory-fresh device they needed to re-register. Making matters worse, many if not most of these users were then unable to register. For some, their Nook Color would no longer connect to any wifi networks, and for others the B&N backend was either overwhelmed or simply broken.
If you were one of the many affected, I am so very sorry for the annoyance and frustration you're experiencing.
If the device didn't do a hard reset and simply de-registered and upgraded the firmware and is simply keeping you from successfully booting into the upgraded OS you should be able to regain access to your files by creating an SD-based OS install using my Guide to installing CyanogenMod 7 on SD card.
You can then browse to your internal eMMC memory and copy any of your data from there. It'll take you about 30 - 45 minutes to make the SD-based install. All you need to do then is browse to /mnt/emmc and that is the root of your internal memory. Most of your files would be under /mnt/emmc/My Files and /mnt/emmc/B&N Downloads.
Let me know if this helps or if you have any questions!
PS - Once you recover your data, or even if you don't, this might be a great time to consider ditching the stock Barnes & Noble firmware in favor of the newer, better, less restricted CyanogenMod 7 (Android 2.3, Gingerbread based) OS. Consider your options by reading this document on what OS installs you can do with a Nook Color.