"At one stage of our Linux adventure we have all come across
this situation. Here you are happily exploring and tweaking your
system when all of a sudden it doesn't start any more. There are
many things that could have been done from changing screen
resolutions to a script or runlevel gone bad.
"If you are from a windows background the most common thing you
have done is to format and re-install from scratch. Those in the
know however know that re-installing linux from scratch is the
absolutely last ditch effort and should only be tried if all other
options have been tried and failed..."