Does a counterspell counter a spell with split second?

Split Second (CR 702.61) is a keyword ability that means: as long as the spell with Split Second is on the stack, players cannot cast other spells or activate non-mana abilities. This effectively "locks" the stack, preventing any instant or counterspell from being cast in response.

Because a counterspell must be cast to function, and Split Second prohibits casting spells while the Split Second spell is on the stack, you simply cannot cast a counterspell targeting it. The Split Second spell will resolve unless it is somehow removed from the stack by another means (such as a triggered ability or a previously-existing effect).

It is important to note that Split Second does NOT prevent triggered abilities from going on the stack, nor does it prevent mana abilities from being activated. Only the casting of spells and activation of non-mana abilities is locked out (CR 702.61b).

Concrete Example: Your opponent casts Sudden Shock (an instant with Split Second) targeting your creature. You want to cast Counterspell in response, but you cannot — Split Second prevents you from casting any spells while Sudden Shock is on the stack. Sudden Shock will resolve and deal 2 damage to your creature.

However, if you had already cast a counterspell before your opponent cast Sudden Shock, and it was already on the stack, Split Second would not affect it — Split Second only stops new spells from being cast after it is placed on the stack.