The rule in the NFL is that if a ball carrier loses his helmet during a play, that play is blown dead. The rule is in place so something like this doesn't happen:
The block occurred during Thursday's 49ers vs. Chargers game. Via SportsGrid:
[Chargers player Mike] Tolbert, who lost his helmet while covering a punt return, sprinted 50 yards downfield in an attempt to tackle 49ers returner Ted Ginn, Jr. As Ginn switched direction, Tolbert pursued, and didn’t see Adams coming from his right side. That’s when the boom was dropped.
As you'll notice, Tolbert was fine -- luckily. What could have been a serious concussion or bloodied face ended in a "tough guy" video.