Comment 9 for bug 571444

This happened to me because for some reason 10.04 does not load the wireless modules prior to attempting to connect to network shares that are in fstab. The network (samba) shares are not able to be found and then the boot just hangs. No shell, nothing, I can only reboot or use the work around, thanks OP for the workaround!

This is an extremely serious bug for any non-power user, as fstab must be edited to fix the initial problem, and it basically makes an upgraded system unusable.