There are many conditions to crash any current market to zero, but in terms of cryptocurrency there are few:

1. War
One doesn't simply recover from a massive destruction, whether by financial, stock or cryptocurrency all will suffer. Rebuilding what has been lost can be a painstaking task. Cooperation and trust are needed very much for any restructure but after a massive war it can be a very serious matter.

2. Network Failure
If the main internet grid has been destroyed or any existing channels of internet has been replace by another technology it can disrupt the current market by shutting down P2P. No connection, No P2P.

3. Global Ban
The current cryptocurrency market possess no potential threat on any fiat currencies but certain countries do ban any transaction in junction with crypto. If there would be a global ban regarding crypto markets and penalize those who engage such endeavors then most likely it will render the market into a crash.