If it's still not showing after you have uploaded the twenty-ten theme, download your theme via FTP to your local computer. Then delete your theme from the wordpress install. This will force wordpress to reload the original twenty-ten theme and not your modified one.
I had to do this the other day when I was testing a new theme I created for my site.
Have you tried using the toolbox 1.1 template from here? It would be a much better place to start as it is an un-style theme, easily customisable to how you would need it to be. CSS knowledge is a must though.
Oh I nearly forgot, if you have your style.css file you can test it by going to the W3C CSS validator as Sixhours posted earlier (link is in Sixhours post). You will need to use the DIRECT INPUT section as you have it on your local machine, this will show up any problems in the CSS. You probably find you have missed either { } or ; somewhere in the file which can break the CSS code in certain situations.
Hope this helps