I tried to update Fedora 17 to 18 from within yum using the directions on the fedora web site. When I rebooted I ended up in the CentOS installer screen. I am not sure how that happend but I can't seem to get out of it into even a basic shell.
EDIT: I managed to solve the problem by backing up the system and reinstalling it
yummethod? Another? – vonbrand Jan 22 at 19:55