I always wonder how things would be reconciled if something similar happened to a bank. What would be the simplest way to ‘download’ one’s balance whilst proving that the downloaded files were signed by the bank?

I'm good with starting from 0 if the bank is (my mortgage is a lot bigger than my balance)

Sods law would be the only bank not affected would be those I owe money to

Your best chance would probably be mailed statements

Your balance is at a bank. If the bank is gone your balance at it doesn't matter.