The most important thing to bitcoin is the internet. At the very worst, If the internet got shut out allover the globe.
1. The nodes wouldn't be able to run/communicate
2. Mining would be impossible
3. Transactions would not be possible
4. The value/demand would consequently drop to Zero as people wouldn't be interested in buying something that doesn't work
5. The End
The chances of the internet being shut-off around the world is pretty much impossible. Internet is sort of decentralized, one country can shut it down but not the entire planet. Making mining impossible is also "impossible" because its highly unlikely for electricity to be shut down everywhere on the planet. People need electricity to survive. Plus there are other ways of generating electricity like diesel generators.
I think the only way how BTC might go to $0 or close is if there is some quantum computer released which can solve private keys or some fault found in ECDSA encryption method. Basically its what is used to secure a BTC address with a public key using a private key.
Who knows, maybe Satoshi sending all his BTC to an exchange can cause it to crash also. All 1 million worth, not enough buy orders out there to absorb that in such a small time-frame.