There is no way to get the pass code back. I'd try sleeping on it overnight to see if you remember, or if you are impatient, power cycle the NC about eight times to clear everything and start over with a "factory-fresh" Nook, which you will have to reregister. The Nook's files will be gone, but any B&n books purchased will be reloaded after you reregister.
To power cycle, press the power key with the nook completely powered off, and as soon as you see the screen "flash", release and then immediately re-press the power key for ten seconds, until it turns off again.
If this sounds difficult, you can take it to a B&N store, where they will perform an equivalent action.
This reminds me of the old joke (slightly modified): "I got a Nook for my kids. I thought it was a fair trade."