If a creature has menace and only one creature can legally block it, does the attack go unblocked?

Menace is a static ability that modifies the rules for declaring blockers. Under CR 702.111b, a creature with menace can't be blocked except by two or more creatures. This means that for a block to be legal, the defending player must assign at least two creatures to block that attacker simultaneously.

When the defending player has only one creature available that could otherwise block the menace creature, that single creature is not enough to form a legal block. Since no legal block is possible, the attacker is simply not blocked (CR 509.1b). The defending player cannot choose to block with just one creature — doing so would be an illegal declaration of blockers.

It does not matter whether the single available creature is large enough to survive combat or has any special abilities. The restriction is purely numerical: two or more creatures must be declared as a group blocking that one attacker, or no block occurs at all.

Concrete example: You attack with a 3/3 creature that has menace. Your opponent controls only one creature, a 2/2 Bear. Even though the 2/2 could normally block a 3/3, your opponent cannot legally block your menace creature with it alone. Your attacker goes unblocked and deals 3 combat damage to your opponent.

This rule applies even if the defending player has two creatures but one of them is tapped, has summoning sickness preventing it from blocking, or is otherwise ineligible — only creatures that can legally block count toward satisfying the two-blocker requirement (CR 509.1b, 702.111b).