Updates that affect /boot do require re-signing, but it shouldn’t drop you directly to a recovery shell, it should indicate that the /boot signatures aren’t valid and ask you to re-sign if you are sure the change was intentional.
Is the RTC time still correct? The date/time appears in the main menu, or date in the recovery shell will show it.
Is it possible /boot was corrupted? You might want to fsck it from a live system. (If you need detailed instructions, let me know.)
In the meantime if you need to use the system and are sure it hasn’t been tampered, you should be able to interrupt the automatic boot and use “Boot options” > “Ignore tampering and force a boot (unsafe)”.
If one of those suggestions fixes it, please let me know so we can improve the diagnostics, otherwise we can start troubleshooting the signature verification to see what precisely is failing.