Well, it impossible to takedown the internet world wide, it would collapse the governments, banks, hospitalsand many more important stuffs.
Bitcoins are hacked in multiple ways and it only happens at the user end not possible on the network level.
It's a bit more dramatic, because this world was once in an era without the internet, yes, more precisely before the internet was discovered as ARPANET using TCP/IP in 1982... it's a bit ticklish because in the years before that, both governments, banks, large companies and hospitals were actually already operating... but it's true that it wasn't optimal... the internet is not the heart of this world, but without the internet life is really very hard..
Some articles that I read, say that Bitcoin can be transacted without using the internet... but I also think about miners, can they mine without the internet? I think the answer is no... the Bitcoin network can only be turned off when all miners are dead... and to kill all miners, just turn off the internet... meaning Bitcoin can be shut down in a very cruel way, namely eliminating global internet services...
As for hacking, it is very difficult... a 51% attack is quite a possibility to hack the Bitcoin network, but it sounds very unlikely considering the large base of miners around the world today.