No CSS FTP?

Hi, I have just started using WordPress. Installed worked fine and I have been working on my site. Occaisionally after I make a change (such as adding a link, category etc) and view in the browser the website will loose all of it’s formatting. Looks like there is no CSS. There is other discussion about this on the forum, but none seem to address this problem specifically. I’ve looked into lots of CSS issues, and am now wondering if it’s something else? Hope someone can help!