Short answers, pulled from the story.
Scholars both inside and outside the USSR attributed the collapse to Soviet-type economic planning combined with political dogmatism. Structural problems included chronic data falsification by bureaucrats, an inability to respond to consumer demand, and a dangerous dependence on oil revenues that collapsed when world oil prices fell in 1986, draining Soviet foreign currency reserves.
By 1970, the Soviet economy was estimated at roughly 60 percent of United States size in key commodities such as steel and coal. In 1989, official Soviet GDP was 2,500 billion dollars against United States GDP of 4,862 billion dollars, with Soviet per capita income at 8,700 dollars compared with 19,800 dollars for Americans.
Oil and gas became the chief driver of the Soviet economy after world prices quadrupled in 1973-1974 and rose again in 1979-1981. Hard currency from oil exports covered food shortages, financed equipment imports, funded the arms race, and underwrote foreign ventures including the war in Afghanistan. When Saudi Arabia flooded the market at the request of CIA Director Bill Casey and world oil prices collapsed in 1986, the Soviet economy came under severe pressure.
Soviet agriculture suffered persistent structural failure under collectivization. Stalin's first Five Year Plan from 1929 to 1933 was a colossal failure, and the Soviet famine of 1930-1933 killed between 5.7 and 8.7 million people. From 1972 to 1986, the Soviet Union failed to produce more wheat than the Western European average, and between 1961 and 1985 it spent nearly 152 billion dollars importing food from foreign producers.
Gosplan was the State Planning Commission, the most important agency in Soviet economic administration. It developed production targets through a process of negotiation between the Politburo, the Council of Ministers, economic ministries, and individual enterprises. Gosplan balanced economic inputs against planned output targets and translated Politburo guidelines into specific quotas for every sector of the economy.
At its dissolution at the end of 1991, the Soviet Union left the Russian Federation with a growing pile of 66 billion dollars in external debt and barely a few billion dollars in net gold and foreign exchange reserves.