Why did Cersei get arrested?

Cersei was arrested by the Sparrows for his sins which include Incest and adultery.

Lancel Lannister became a devoted sparrow. He told the Sparrows about Cersei's relationship with Jaime and her past relationship with him.

The Sparrows were empowered, given weapons and allowed to take people prisoners if they had faith problems. Cersei was the one who empowered them yet they threw her in jail. See also why Cersei hates Tyrion.

The Sparrows don't recognize any authority. To them all people are considered equal in front of God.

King Tommen didn't want his men to attack the Sparrows. So both Cersei and Margaery remained in jail for sometime.

Kevan Lannister Knew that Cersei is in jail but didn't want to visit her. See also summary of Episode 8 season 5.

Cersei was given the option on daily basis to confess for her sins and get released. Cersei refused to confess. See also why Tywin hated Tyrion.

King Tommen got depressed and isolated. He didn't know what to do and he remained in his room. This allowed the Sparrows to keep Cersei jailed.

Because of the way she used to deal with people Cersei had few friends outside who were willing to save her from prison.