Czar Nicholas II abdicated the imperial throne in 1917 for a couple of different reasons which began long before the year of his retirement.
Russia was a poor agrarian nation with nearly 80% of the population living as peasants while the top 1% of society controlled nearly a quarter of the countries total wealth. Although most peasants were free to buy their own land, the requirements and fee's were sooutragous most were unable to improve their lot in life.
Russia was also seen as technologically backwards nation that had not modernized at the same rate as other European nations. In 1904 Russia was humiliated when Japan defeated the country in a terribly one-sided war. This defeat by a non-European nation hurt Russia's standing with other countries.
Circumstances like these led to the 1905 Revolution, where both elements of the military and workers revolted and brought the country to a standstill. Nicholas tried to stop the strike from spreading by sending his soldiers to fire upon demonstrators, an episode called Bloody Sunday. This helped to fuel the fires of revolution rather than extinguish them.
Nicholas also failed to institute reforms that would have helped stave off a furthur spread of anti-Czarist anger when he released the October Manifesto to the Duma, but although he swore publically that he was committed to revolutionary reforms, he unfairly curtailed the power of this law-making body to preserve his own power.
But of all these events, the one that proved to be the biggest nail in the Czar's coffin was Russia's entry into World War I. Initially a popular move, Russia lost over 1 million soliders in the first year alone. Without workers, farming and industry failed to produce resulting in massive starvation. Nicholas left for the front to try and bolster moral, leaving his German-born wife to rule in his stead, a very unpopular move.
Riots began. Order broke down. Revolutionary parties began to take over and finally, with authority non-existant and thousands of soldiers joining the revolutionaries, Nicholas abdicated. Eventually he and his family were killed.