One possibility is that the user-adjustable display contrast is set too low. It can be adjusted by pressing and holding down the ON key, then pressing the + or - key to raise or lower the contrast, respectively. In an emergency, a master clear (press and hold down ON, then press and hold down A, then press F, then release all three keys) clears all main memory but not port memory, and resets the display contrast to its original factory default.
Another possibility is low batteries, maybe.
Disclaimer: I don't work for HP, but I enjoy using their calculators.