We can imagine a war that causes a global blackout. So if the hash rate drops by 50%, assuming the next BTC difficulty adjustment is in 2000 blocks.
As the hash rate drops by 50%, each block will be mined in 20 minutes instead of 10 minutes. So, if the hash rate drops by 80/90/95%, it is possible that there will never be a new BTC difficulty adjustment because the time to find a nonce matching the current difficulty will increase. If my calculs are correct, for a drop of 95% of hashrate, each blocs will be mined in 200 minutes, so 277 days to reach a difficulty adjustment (considering 2000 blocs remaining).

There are an error ?