I seriously don't know what key I hit, but it turned off the hub for the gun in BF3. The hub located in the lower right corner is now gone, and I have no idea how to bring it back. I've pressed every key on the keyboard without luck.
The simple answer is that you've joined a hardcore server. The hardcore rule-set makes it so you can't see your ammo or your health.
If this isn't the problem then it is unlikely that you've hit a button by accident as this isn't a feature you can simply toggle on and off with a hotkey, it is likely a corrupted installation that can be fixed by reinstalling the game or perhaps your GPU drivers.