Oil was 'only' 12% of GDP in 2013, so a 50% reduction of the GDP since then can not be explained with a reduction in oil prices.

Venezuela remains highly dependent on oil revenues, which account for roughly 95% of export earnings, about 45% of federal budget revenues, and around 12% of GDP.
https://www.cia.gov/library/publications/download/download-2013/index.html

Venezuela has a majority state run >50% (the only one in Latin America?) economy, highly susceptible to change in political leadership.