Even in a country that for decades has been accustomed to frequent outages amid a series of economic crises, the grid failure was unprecedented in modern times,
[…]
They also blamed breakdowns in old thermoelectric plants that haven’t been properly maintained because of a lack of hard currency due to U.S. sanctions, as well as insufficient fuel to operate some facilities.

The blockade is clearly the problem. Cuba is under siege and this is an obvious result.