I just finished going through the upgrade to 22.04, as prompted by the OS. The upgrade completed successfully, & prompted me to reboot. But after a reboot…the system fails to startup.
If I just let it boot the new default “Neon GNU/Linux” entry it added to the top of the grub menu, it stops saying “Gave up waiting for root file system device. Common problems: […],” with a line at the bottom “ALERT! UUID=xxxx does not exist. Dropping to a shell!”
If I reboot & manually select the grub menu item I was previously using, it just stops at a BusyBox prompt with no error or information.
Not sure why a simple upgrade would hose my system like this…?
Edit: I’m using full disk encryption (as was setup by checking “encryption” in the Neon installer when originally installed).
A full release upgrade, such as from Ubuntu 20.l04 to 22.04 is not really “simple”, to begin with. Sometimes a full upgrade like this can fail, for a variety of reasons. The addition of waiting for over 18 months to upgrade may be introducing extra complications that are going to be untested.
However, the error you get is quite useful - it is saying that there is something wrong with one of your drives or partitions - the UUID that corresponds to a device isn’t there any more. Which could mean an issue with the drive, or for some reason the UUID has changed.
Hopefully it is just a messed up UUID, and not a hardware issue. You may be able to fix it usintg Boot-repair
Thanks for the super quick reply 
So after the upgrade failed (and since, as you noticed, it’s been quite some time), I decided to just try a fresh from-scratch reinstall (my important data is all on a separate partition, so this one was OS-only).
Unfortunately, even a fresh install seems to be failing. I documented the steps in detail in this other thread: Fresh Neon install can't boot if encryption is used (20240201-0717 iso)
If you have any ideas there, would be greatly appreciated. It seems like the 22.04 installer gets messed up when using full disk encryption (where the 20.04 worked fine). Really hoping I can avoid just reinstalling the same old 20.04 version I’d been using previously, haha
Also, for anyone else who comes across this (though I’m now personally no longer trying to upgrade - just trying to get a fresh install to work) - I found this: https://www.reddit.com/r/kdeneon/comments/ycpmg6/fyi_be_careful_when_upgrading_to_kde_neon_with/
I find that this can be handy at times to get the disk info I need to troubleshoot issues.
sudo lsblk -o path,label,uuid,fstype