I'm having a bit of a problem with my NOOK Color. It has not been turning on even though I hold down the power button. [...]
Have you tried the long-press of the power button twice? What commonly happens is that the device is "on" in standby, but the screen doesn't wake up. The first long-press (20+ seconds) turns it truly "off", the 2nd turns it back on for a reboot.
It sounds like you're experiencing the "sleep of death" symptom. Next time it happens, try taking it in a dark room to see if there's a soft glow. Have you rooted or are you running alternate firmware?
Don't worry too much. The hardware and battery seem sound. You may have to do a reset at worst to fix it. The NC is a pretty robust device.