What was the reason Dr. Mannette was imprisoned?

Dr. Mannette was imprisoned in the Bastille, a notorious fortress prison that stood as a hated symbol of pre-Revolutionary France and whose fall is generally thought to represent the start of the French Revolution. He rotted away in that hellhole for eighteen long years: his lengthy period of incarceration taking a terrible toll on both his mental and physical health.
Dr. Mannette was imprisoned for informing on Darnay's father and his uncle, the wicked Marquis St. Evrémonde. They were responsible for the rape of a peasant woman and the murders of her husband and brother. The Evrémonde brothers obtained what's called a lettre de cachet to keep Dr. Mannette quiet and send him to prison.
A lettre de cachet was a sealed letter from the king that contained his express orders, normally relating to the punishment without trial of certain individuals. There was no appeal against a lettre de cachet, and they were wide open to abuse, often used to settle scores or personal vendettas. That's precisely what happened to Dr. Mannette. Two aristocrats were able to get away with breaking the law; not only that, but they were able to send an innocent man to prison for eighteen years, and all because he had the courage to do the right thing. It's no wonder that the revolutionaries hated the Bastille and the manifestly unjust system it represented.