Haven't these happened already? A couple of years ago, China and other countries banned bitcoin but people no longer remember those moments because of how quickly everyone who had doubts saw how bitcoin strived in those situations. The only reason why most countries are beginning to adopt bitcoin in exponential rates is because every attempt to try and stop bitcoin only led to more growth so they just joined the bandwagon instead.

TLDR: What happens if all countries banned Bitcoin? The network operates as usual. The chain moves forward through transactions.