Check if you have enough free memory space left.
Linda, what geertm wrote.
Just go to Settings and click on Device Information.
There are two rows. One for
Storage for B&N Content
and another for
Storage for Other Content.
I think the first is where Overdrive saves its files.
Try archiving the app and then unarchiving it. I was able to download books after that.
I thought you said you can download to PC and then transfer to your Nook. Is that no longer working?