I just upgraded from Ubuntu 9.10 to 10.04 and immediately had some problems with booting after a restart.
Here is a video but I will explain it anyway:
The font changed suddenly during startup, and then the screen flashed, and the logo appeared but then became what I would call pixelated. Then all I got was a black screen and it just hung there.
Did a hard restart and entered the GRUB loader and chose Ubuntu 10.04 kernel 2.6.31-21 instead of Ubuntu 10.04 kernel 2.6.32-22 and it came up with this message which hung there long enough for me to snap a photo:
mount: mounting none on /dev failed: No such device092b3250a
chroot: cannot execute /etc/apparmor/initramfs: No such file or directory.