Soak the hen in a warm Epsom salt bath for 20 minutes. Dry, then warm her in a quiet box. Lubricate the vent with KY jelly or vegetable oil. Give a calcium boost (crushed Tums or liquid calcium). If she has not laid in 6 hours, call a vet.
Prevention
Feed a balanced layer ration with oyster shell free choice. Avoid sudden lighting changes that trigger early or oversized eggs. Manage weight, obesity is a major risk factor.