Has the cat got your tongue?

Meaning: something that you say to someone when you are annoyed because they will not speak.

Example: Well, has the cat got your tongue? I'm waiting for an explanation.

let the cat out of the bag
 
Meaning: to tell something that is a secret, often without intending to.

Example: Amazingly, not one of the people who knew about the surprise let the cat out of the bag.