If a monster cannot find a valid focus because all attack hexes are occupied or unreachable, the monster does not move or attack but still performs any other abilities listed on its ability card.

This is supported by the following rule:

• "It is possible that a monster is unable to find a focus if it cannot reach a valid hex, given infinite movement, from which it can perform its attack. In such cases, the monster does not move or attack but still performs any other abilities listed on its ability card."
  (page 41)

Additionally, an example is provided:

• "The Algox Guard 1 cannot find a valid hex from which to perform a melee attack, because all hexes adjacent to the Blinkblade are occupied or invalid, so the Algox Guard cannot find a focus and does not move."
  (page 42)

Summary:
If a monster cannot find a valid focus due to all attack hexes being occupied or unreachable, it does not move or attack, but still performs any other abilities on its ability card.

Sources:
page 41, page 42

Source: Core Rulebook (pp. 41, 42, 74, 75, 76)