A few things to try.
1) Firstly, since you said 'recently' my advice would be to go to a store and get a replacement.
2) Secondly, you can try the following:
Power Off: Press and Hold power button for 20 seconds.
Power ON: Press and Hold power button for 4 seconds.
3) Thirdly, you can try (please search on forum threads for exact steps here)
Press and hold power button and N button for 20 seconds.
Note: I'm not sure what kind of reset this does. It might do a factory reset. Which means you'd lose data. So please search and check first.
4) Let the Nook charge for a few hours. Ignore the screen flashing OR turn off Nook and then charge.
You could also wait to see what other people suggest. Lots of smart people on this forum. I see it's only been 20 minutes since you asked your question. If you want 3-4 hours you'll get some very solid advice on what the fix might be.