Interrupting an upgrade may leave your OS in corrupted state.
No one can tell you how to fix it; indeed most of us would just restore from backup or do a fresh install.
In fact there is NO RECOVERY partition on a normal Raspberry Pi OS OS.
If you want to attempt recovery of your data use a fresh SD Card.
"how would I attempt recovery with a fresh ...
You've asked a good question - one that's a potential concern to all RPi users. I'm still working through this - I may not have a complete answer now, but I'll post what I've learned now, and update when I gather "the rest of the story". Feedback, corrections and comments are welcomed.
Some clarification on semantics is always useful when ...
apt should be prompting you to accept the changes. If not, you can run apt with the additional option apt-get --allow-releaseinfo-change update
This is due to the new release of Debian Bullsye which happened a couple of weeks back.