I wanted to upgrade from 11.10 to 12.04 with the built-in upgrade-feature. but during the upgrade my pc stopped working. When I restarted my PC Ubuntu starts OK, but with many errors (many apps don't work and some graphic-errors). I try to upgrade again to 12.04, but I can't, because my Ubuntu identifies itself as 12.04. I tried to upgrade with the cd-setup but this also doesn't work. What should I do, if I don't want to reinstall the whole system?