If you started a partial upgrade (through the package management system, apt) to the next version but stopped it before it finished (but after it started, i.e. after packages were downloaded and the actual upgrade process began), chances are that the "important" parts of your system were upgraded only partially and thus you cannot boot your system unless you either reinstall it from scratch (clean installation of either version) or make the upgrade process go through. To try the latter one you'll need a livecd (or something equivalent) if your system cannot boot. From the live system, you will have to mount your installed system partition(s) and use chroot to target the changes to them (and not the livecd operating system), then restart the upgrade process. It may or may not continue/help you out, depending on where exactly the upgrade stopped. Personally I've been able to finish a halted upgrade that wrecked support for network cards and whatnot by downloading the new version as a cd image (Ubuntu: alternate disc, the "desktop" version won't do) and writing it on disc, then telling the system to look for upgrades from that disc rather than trying to use internet reposities.
The easiest way is to just do a clean installation over the old (wrecked) system and pull your personal data from a backup. You should always have a backup (or many) available.
Edit: noting the version numbers you mentioned, I see where the problem might come from. You said you have used 9.04, which you tried to upgrade directly to 10.04, thus skipping over the version 9.10. Generally speaking this doesn't work. You are, if you do an upgrade of an existing system instead of a clean installation, supposed to upgrade to the next version, not anything beyond it. So if you wanted to upgrade from 9.04 to 10.04, you'd need to first upgrade from 9.04 to 9.10, then after that upgrade from 9.10 to 10.04. This means downloading and installing the packages two times, which is time consuming. Especially if you have a separate home partition, you'll get through much more quickly if you simply install the newest version over the old one (but not formatting home partition).
Last edited by b0uncer; 09-23-2010 at 04:44 AM.