Venezuela was having serious issues even before the price of oil took a nose dive. One example is the food shortages. They were happening well before the crisis. Centralized food markets with price controls always creates shortage. It's simple economics.

If your economic system can't efficiently feed people, more complex industries will be even harder. Not to mention building new industries.

The failures of their socialist experiment go far deeper than simply a non diversified industry. The drop in the petro industry showed immediately how fragile the massive spending schemes and nationalization was.

As Thomas Sowell said (paraphrasing), the problem with redistribution is that you can only take someone's money/property once, you can't take their ability to continually produce goods. So when that well dries up the cracks in the system become apparent.

The wikipedia article on the Venezuelan economy is a good read about the risks of centralized planned economies.