There are a lot of things. Even before Chavez, the economy wasn't especially great. The dependence on oil has been a problem since early XX century, but obviously that isn't enough to destroy a whole economy. Since the dictatorship of Marcos Perez Jimenez ended in 1958, corruption has been a huge problem in Venezuela with the two-party system that the main right and left wing parties agreed to maintain. In the 70s and 80s, the country had a crazy amount of money thanks to the oil, but that didn't stop the debt from increasing, there was a lot of missing money and projects that were never finished (but the money was already approved by the state) and when they tried to do something, the country was already in ruins and that's when Chavez wins the elections.

When Chavez won, he considered himself as leftist, but not as socialist (there are even interviews where he say that socialism doesn't work and Cuba was a dictatorship), he was very moderate. After some time he started with his socialist campaign, but it wasn't until he won his second election that things started to really go wrong. That's when he started with most of his nationalization and expropriation of private companies, which, most of them ended in the bankruptcy after that and just stopped producing anything, by 2010 there were already hundreds of companies that were nationalized and ended producing nothing, which just helped to strengthen the monopoly that already existed in the country (there were moment where most of the food products you could find in stores, where made by the same companie: "Polar", which is still a private companie). In his second term, he also started giving a lot of money and oil to Caribbean countries (mostly Cuba) and Bolivia, to help develop those countries energy. All of this didn't hit too hard at the beginning, because the country still had a lot of money from oil that made possible to grant money in subsidies, but a lot of workers from PDVSA (the state oil company) were already gone and were replaced for people loyal to Chavez, which made PDVSA start to decline. Like in the XX century, there were also a LOT of projects that were never finished but the money was already gone, or they were finished but never worked for too much times (like the multiple companines of Cuban icecream "Coppelia" that were built in Venezuela, but stopped producing after a few days of starting to produce or the huge amount of Chinese Yutong buses that were bought for a lot of cities, but stopped working after a few years and now you only see those buses working for political events). Around 2012, the huge crisis started, there were already a lot of basic products lacking from the stores and the bolivar worth less everyday, then Chavez died and the crisis started to get more international attention and things just got worse and worse every day, but the state didn't stop giving subsidies (there were things like CADIVI that gave grants to travel to other countries or buy stuff online) even when the country was producing almost nothing and the oil production (and price) wasn't like before. The situation just kept going and the lack of products was huge until they started to bring more products from countries like Brazil, Mexico or Middle East countries like Turkey, but they were usually overpriced or really low quaility. In 2017 there were sanctions by U.S and E.U to some companies, but by that time Venezuela was already producing almost nothing (Venezuela was already buying gasoline from other countries) and the debt with China was huge, which Venezuela is forced to pay with oil and in other ways (there were Chinese groups extracting raw materials in Venezuela and stuff like that) .

So, in short words: corruption, oil dependance, monopolies created by the state and destruction of Venezuelan companies.

Sorry if my English isn't that good and if I say too much stuff.