Two Silverblue systems here got hit by the secure boot signature bootloader bug on the upgrade that brought in a new kernel. Good thing there are rollbacks (to boot an older version) and a workaround.
Issue: https://github.com/fedora-silverblue/issue-tracker/issues/543
Workaround to keep using secure boot (run these commands on a working boot): https://github.com/fedora-silverblue/issue-tracker/issues/543#issuecomment-2048350047
Hopefully there's an automatic fix soon? ('bootupd' should be the long term fix for this.)
@garrett this is me screaming.