Okay. As I had this problem this morning, and it took me forever to find an answer, I figured I might as well share. For people who woke up to find their override coding all messed up, from the LJ Support page:

For security reasons, we have made some changes to the HTML and CSS cleaner, a routine which strips invalid, incorrect, and possibly malicious code from journal styles and overrides. If your journal is displaying improperly, you will need to correct the errors in your style or overrides. LiveJournal support cannot offer one-on-one customization help; you should perform a web search for a CSS tutorial.

everything_lj may provide some tips on how to fix your invalid CSS, and there's a CSS validator here (although, if you have a colored scrollbar, that will read as invalid, because by W3 standards, it is. But LJ will parse it fine, because it's valid in IE). Also, if I coded your layout, lemme know it's having problems and I'll fix it - I tend to reuse a lot of the same basic override code, and there was one minor bug in my own set that was screwing around with some stuff.