Why did tsar Nicholas II had power?

Answer 1

Nicholas II believed he had an obligation as Tsar to retain complete autocratic control over Russia as all the Tsar's before him had done. Both he and most of the peasant civilization of Russia believed that the Tsar was practically sent from God. Nicholas II believed he had to be true to that concept.

Answer 2

Nicholas II had power because he inherited a monarchy. Unfortunately for him, it was a monarchy in decline and he was ultimately overthrown when Nicholas II lost the backing of the army and police forces in Petrograd during the riots now called the February Revolution.