II Kings - Chapter 11
And One Queen
Atalya, mother of Achaziah, saw her son was dead and took the opportunity to eradicate his heirs, her own grandchildren. (Remember, she was the daughter of Ahab and Jezebel - the apple doesn't fall far from the tree!) She wiped out all the descendants of Solomon, through whom the throne is transmitted, except for one. This one was saved by Yehosheva.
Who was Yehosheva? She was: Yehoram's daughter, Achaziah's sister, wife of Yehoyada, the Kohein Gadol (High Priest). She was (possibly) Atalya's own daughter.
So, Yehosheva took her infant nephew Yoash and hid him from Atalya. He was hidden in the Temple for six years, while Atalya usurped the throne. In the seventh year, Yehoyada called together the leaders of the people and revealed to them that a rightful heir was still alive. It was time to restore him.
Yehoyada divided his forces so as to protect the young king from any who would harm him. With everyone in place, Yehoyada crowned Yoash with David's ceremonial crown (see II Samuel 12) and gave him the Torah scroll that all kings must carry. The people cheered, "Long live the king!"
Atalya heard the commotion coming from the Temple and went to investigate. When she saw what was going on, she tore her clothes and shouted, "It's a coup!" (Like she should talk!) Yehoyada had Atalya arrested, so that she wouldn't be executed in the Temple. She was marched through the back streets to the service entrance of the palace and was executed there.
Yehoyada then forged a covenant between the people and G-d, and between the people and the king. The people tore down the temple of Baal in Judah and smashed the idols. They marched the new king to the palace via the public thoroughfare and placed him on the throne. The Navi doesn't tell us any specifics of Atalya's reign, except that the people rejoiced that it was over.