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Acts 5:19-20 (KJV) But the angel of the Lord by night opened the prison doors, and brought them forth, and said, Go, stand and speak in the temple to the people all the words of this life.

Acts 12:7-10 (KJV) And, behold, the angel of the Lord came upon him, and a light shined in the prison: and he smote Peter on the side, and raised him up, saying, Arise up quickly. And his chains fell off from his hands. And the angel said unto him, Gird thyself, and bind on thy sandals. And so he did. And he saith unto him, Cast thy garment about thee, and follow me. And he went out, and followed him; and wist not that it was true which was done by the angel; but thought he saw a vision. When they were past the first and the second ward, they came unto the iron gate that leadeth unto the city; which opened to them of his own accord: and they went out, and passed on through one street; and forthwith the angel departed from him.

Missing from the angel's instruction to Peter on the second occasion when Peter is miraculously released from his prison cell is the instruction to "speak in the temple to the people".

By this time, Israel had fallen (Acts 7), Peter had been shown that God was now operating in the world through his grace and without distinction between men, and the Body of Christ was established with a base at Antioch (Acts 10-11).  The gospel of the kingdom was for now to be preached no more. 

 Peter's message was no longer relevant to unbelievers, so he was not told to preach it. 

And he didn't. Instead, having shown himself to others members of the church, he went into hiding (Acts 12:17), and is only heard of once more in the book, in Acts 15.