OP, in case you still haven't given up I think I can fill in the gaps. You got a lot of advice somewhat in the right direction but no one telling you how to actually sort it out I think.

It's likely that your /dev/sdb2 is now either missing (bad drive or cable?) or showing up with a different name.

You want to update your fstab to refer to your root (and /boot and others) by UUID= instead of /dev/sdbX. It looks like you are not using full-disk encryption but if you are, there is /etc/crypttab for that.

First off, you actually have two /etc/fstabs to consider: One on your root filesystem and one embedded into the initramfs on your boot partition. It is the latter you need to update here since it happens earlier in the boot process and needed to mount the rootfs. It should be a copy of your rootfs /etc/fstab and gets automatically copied/synced when you update the initrams, either manually or on a kernel installation/upgrade.

So what you need to do to fix this:

1. Identify partition UUIDs
2. Update /etc/fstab
3. Update initramfs (update-initramfs -ukall or reinstall kernel package)

You need to do this every time you do changes in fstab that need to be picked up in the earlier stages of the boot process. For mounting application or user data volume it's usually not necessary since the rootfs fstab also gets processed after the rootfs has been successfully mounted.

That step 3 is a conundrum when you can't boot!

Your two main options are a) boot from a live image, chroot into your system and fix and update the initramfs inside the chroot, or b) from inside the rescue shell, mount the drive manually to boot into your normal system and then sort it out so you don't have to do this on every reboot.

For a), I think the Debian wiki instructions are OK.

For b), from the busybox rescue shell I believe you probably won't have the lsblk or blkid like another person suggested. But hopefully you can ls -la /dev/disk/by-uuid /dev/sd* to see what your drives are currently named and then mount /dev/XXXX /newroot from there.

In your case I think b) might be the most straightforward but the live-chroot maneuver is a very useful tool that might come in handy again in other situations and will always work since you are not limited to what's available in the minimal rescue shell.

Good luck!