If someone takes a head wound in a game, there are all sorts of dramatic things that can happen. In a light game you could simply have them see cartoon-like stars, or little birdies, floating around their heads. You can go for the dramatic and rapid growth of a lump on the head (covered in crossed bandages in the next scene).

More seriously, you can have the victim lose their memory. There's complete cinematic amnesia, where they have no idea who they or their friends are. Or maybe they remember most things but forget one important fact they should know or thing they are supposed to be doing.

Then there are other effects, such as loss of vision or hearing, or dizziness, or loss of balance and coordination. Basically, you can justify almost any game mechanical penalty you can imagine from a blow to the noggin.

Players: Consider a helmet.

Transcript

Han: I've almost got the coin. It's just under these wires.
[SFX]: bzzzt
Han: Oh. There seems to be a small complication.
Han: Where's that wire splicer?
[SFX]: bzzzt! bzzzt!
[SFX]: fzzzt
Han: Hmmm. My complication had a little complication.
Han: Chewie, hand me the entire toolbox.
Chewbacca: Be careful, old chap, it's balanced precariously right above your head.
Han: {from below} Wait, if you're here, does that mean nobody's flying?
Leia: Oh look, a thing I need to dodge.
[SFX]: jerk!
[SFX]: thunk!
Han: Ow, my head!
Han: My god, it's full of stars.