If the Internet shuts down for just a week, be rest assured Bitcoin will survive it and if it goes beyond that, I can't tell exactly but I believe possible solution will be formulated but it will take some time, it may not be immediately but it may happen just like we had with COVID 19 years back, it was a global epidemic without a cure as at the time of outburst but after awhile it got controlled and today it's not one which brings things a halt again as before.
There is nothing to formulate, the solution already exists and is a feature of the protocol.
Miners keep mining. Longest chain wins after a week.
There it is.