The story comprises all of Acts 16. There were some local schemers who were making money off of a bedeviled girl who was a soothsayer. Paul called the evil spirit out of her and there went the schemers source of income. Of course, they were irate and went to the local magistrates and reported them as being unlawful and Romans to boot. They incited enough people that Paul and Silas were whipped and thrown into prison.
Paul and Silas, being missionaries, did what missionaries do. They began praying and singing praises to God. It's midnight, everything is quiet except the worshiping heard up and down the prison. Then suddenly everything changes. Everyone in the prison is no longer bound. The doors fly open. They can walk out if they so desire. The guard would rather kill himself than be tortured for not keeping them imprisoned. But, lo and behold, Paul and Silas shout out to him that they have not gone. The prison guard, so moved by the events, is saved.
There are other "suddenlys" in the Bible. One is in Daniel 5--the hand that suddenly appears and writes on the wall and dooms the king of Babylon. He was slain that very night. In Matthew 25 Jesus gives the example of five foolish virgins who didn't bring oil for their lamps. When the bridegroom came, they weren't prepared and opportunity had passed them by. Verse 13 of that chapter says to watch (be ready) because no one knows the hour or the day that the Son of man will come. First Corinthians 15:52 it says, "In a moment, in the twinkling of an eye...". That's pretty sudden. Life is going to change that fast--probably faster than you can snap your fingers. Only God can do something like that. We must be ready for a suddenly. Keep our lamps trimmed and full of oil. Jesus will be here suddenly and ready to receive the ones who are prepared.