Have you revently hit level 15 and unlocked the weapon switching? If you don't have anything on your second bar but have a second weapon equiped you can switch and see a blank bar.
If you're level 15 or over, you might have hit the tilde key ~ accidentally while playing (if on the PC), which switched your weapons bars from set 1 to set 2. The first time I did this, it was accidentally during a fight, and suddenly all my abilities seemed to disappear and I didn't know what had happened. You press ~ again to switch them back.