A few decades ago, China was at most considered a country starting to develop itself, but usually even a third world country. If a person 30 years ago would have heard that China nowadays would be a primary economy and would likely become the first world economy in little time, he'd likely have considered it a nonsense.
But in very little relative time it has developed economically in an extremely spectacular way, I'm no expert in history but I think there wasn't even any empire in history that grew in importance as fast as China.
Which are the main reasons it was so underdeveloped economically some decades ago, specially considering that either their workforce or social mentality coudn't change too much in so little time?
Edit: I just want a brief overview of the main causes that caused that underdevelopment. I obviously do not expect a full detailed answer.