I haven't had this problem with my Nook HD+, but something similar did happen with my Android phone. In the case of my phone I unmounted my SD card & remounted it - which also involved turning the phone off & restarting. When it remounted the SD card, it could read it again, and then decided that it couldn't. I left it alone while I tried to figure out what to do, but whilst I did this it just started to read it ok again, and has been fine ever since. The problem seemed to coincide with the phone switching iself off because of low battery, but no idea why that would be an issue.
So in the case of the Nook, I would recommend powering it down and then powering it back up again. (That's pressing the side button for 6 seconds til it asks if you would like to shut down).
If this still doesn't work, it may be that a file on the card has become corrupted and is giving your Nook probems reading the card.
So that might need you to reformat the card (settings>all settings>storage management>SD card>Erase SD card) and reload the music.
If that still doesn't work, it might be an issue with the SD card itself.
Nook recommended cards are scandisc class 4 microsd, and people have used other classes and other brands with varying levels of success. It may be worth trying one of these if you have one, or using the searchbox to see if using your brand gets a mention.
biemer, adding to what Albion Rose wrote,
I've seen certain problems happen on devices ONLY when battery is low.
SD Card I've seen problems that go away magically.
I think the best options to try are
1) What AlbionRose suggested.
2) Get a Card Reader for your PC and format card on PC.
3) Restart Nook and try card again.