Apparently there were some glitches in Barnes & Noble's system over the weekend, and folks who downloaded free ebooks were charged regular price for them. Keep an eye on your receipts and your accounts if you downloaded anything this weekend! I was charged for six free ebooks, myself.
The good news is that apparently they are already aware of the problems over the weekend, so if you do have to call, they should give you a refund (or refunds, as the case may be) without any trouble.
If you need to call Barnes & Noble's customer service, the number is 1-800-THE-BOOK. You can also email B&N customer service. I've only ever dealt with them via email (yet — we'll see if I end up needing to call regarding these refunds) and they are usually pretty good at responding. Depending on who gets your email, though, sometimes you get some really lame form letters in response.
I've also had to deal with both Kobo and Amazon (Kindle) customer service in the past, both times to request refunds, and both of those went exceptionally smoothly. Amazon especially was very fast, with less than a 12-hour response time. As much as I hate Amazon, I have to give them credit for that!