One known cataclysm that could make the whole interruption of all the telecommunications but also every electricity network is a Major big Solar storm that hits the earth!
But what about Bitcoin..? In such case the bitcoin network would be paused, not all at once., some nodes before others.. Gradually, following the interruption areas of the internet!
However, after all is reestablished, the normal life resumes and Bitcoin actors want to relaunch the Bitcoin network.. So first: not everybody have the same copy of the blockchain as some nodes continued recording after the others got interrupted ! And what if some entities wanted to fool the word creating completely artificial blocks and adding them to the blockchain ? How to trust a copy of the blockchain then ? Do all late copies should not-trusted/canceled ??
So what of a code would be part of Bitcoin protocol and that would depend on that the network is running that could sign blindly the blocks.. (Putting like a seal on them), include in the blocks a cryptographic signature that works like authentication signature ?
I think it would be good if you start it by explaining what damages can Solar Storm cause to earth in more details but let's take significant solar events as an example. As far as I know, even in the events of significant solar storms, the whole world won't lose the electricity and internet fiber optics will be safe (correct me if I'm wrong). So, since Bitcoin is decentralized, it will survive because some countries will have electricity and they'll be able to keep Bitcoin nodes and miners running. Many people all over the world have solar panels, they'll also be able to run Bitcoin nodes but I think that trading will take a huge hit because these platforms might close for a while and price might significantly crash.
By the way, I'm sure that Bitcoin itself is safe. Even if someone tries to use the advantage of the situation and fork Bitcoin, it will still be safe because every bad action can be reversed once the whole world will return back to normal life.