Can't alter CSS all of a sudden

I’m pretty new to WP and CSS but quickly learning.

I’m trying to make a few changes on the Twenty Ten theme (v 1.2) and for some reason it no longer lets me alter any CSS.

It doesn’t matter which bit of the coding I try to change, but every time I hit save it brings up the style.css and from then on whatever I click it just stays with style.css showing

The only way I can get off it is to close my window and then log back in.

Any suggestions?