Well, simple. Bitcoin is gone forever if block chain get deleted... but is it even possible?
EMP strikes knocking out power to the entire world wouldn't do it, so no, not really. I hear there's a blockchain in space being worked on, too.
EMP would not even affect the existing copies of the blockchain as it would only make the computers unusable, it would not delete the information contained on the computers.
This plus the fact that there are many computers (some of which likely are running as a full node - a computer that will contain the full block chain) that are somehow protected from EMP via shilds