Hi Melvin! Were your ebooks purchased through Nook, or did you load them on yourself?
As long as the titles haven't been deleted from your Nook account, we should be able to re-download them to the device. Please try this:
Refresh your Library by tapping the Overflow Menu at the upper right corner of the screen and then selecting Refresh from the menu. This will sync your library to the cloud for any recent changes to your Library.
Do your books sync back to the device after this?
Hi there and welcome
This is usually an issue with the Firmware, so I would first like you to do this please.
Remove the charger from the Nook and hold the power button to off for 35 seconds.
Release, then connect the charger and try to turn on the Nook and access your books.
Let me know the results please