So i've heard theres some problems with upgrading from bionic to cosmic. One of them was that luks and grub was changed a bit so that some systems wouldn't boot. If it helps, i installed lubuntu using the option to use the entire disk and encrypt it so it automatically partitioned the disk. I don't want to upgrade unless i know its good to do so.
tl;dr For me, apparently
cryptsetup went missing during the upgrade. So make sure it's installed, then, do
sudo update-initramfs -u -k all to ensure that you don't end up with a system you cannot boot.
I had this kind of problem the rumors are about ;) Rebooting an old kernel worked first, but after the next
update-initramfs, booting any kernel would fail:
WARNING: Failed to connect to lvmetad. Falling back to device scanning.
Volume group "xubuntu-vg" not found.
Dave up waiting for suspend/resume device
I fixed this by
chrooting into the broken system, installing
cryptsetup, which apparently went missing during the upgrade (!!), and then properly doing
chroot. This is described here (unfortunately german, and when you follow it, make sure
boot is mounted), but hopefully, it's not necessary when you fix the system before rebooting.