It does sounds like your CSS file might be missing, it's hard to tell without actually seeing it though. If you wanted to test to see if this is your problem, you could make a backup of your /styles/phplist.css
file, then grab a default copy of phplist.css
and put that in place and see if it makes any difference.
If you private message me with some further details I can take a look at see what else it might be.