Do not interrupt. Let the student complete the answer. If nothing works, then give the answer yourself and ask the student to restate the answer.
If that does not help, then rephrase the question. Do not ask another student to answer and ask the student to repeat the answer.
Break up the question into small parts. Encourage the student. Do not humiliate the student for not knowing the answer.
Restate the question. The whole process should not take too much time, about 15 seconds. Or else, the class may become inattentive.
Simplify the question. Give hints.