If you are using default ultra96 image from sd card try to reflash it. Also try to connect uart and look into console is it printing something? It happend few time to me when image in sd card got corrupted due to inappropriate shut down.
I second what mindaugas says. Try reflashing your SD Card. If you find you do need an RMA, I believe the standard board warranty is 45 days. You'll need to get back in contact with whichever organization from whom you purchased the board.