Web Design after I edit my the psd the css goes to crap

After I edit the .psd the css goes to crap

ok so I downloaded the "music store" template and if I run the file in golivecs2 it all works great. all the buttons are links and the mouseovers are fine.
same if I access the html with any browser.

once I edit the psd and change things around (no slices touched mind you)
and, once the html is updated, all the css and links are screwed up.
if I try and link to the external stylesheet (the one included) it screws up my new headders graphics and none of the mouseovers do squat. in fact none are even links anymore.

Where have I gone wrong here?
absolute confusion.