problems with css on any theme, css not working properly.

I’ve been having many problems along my css editing. First I tried with a css theme not from wordrpress.org then i browsed and installed a theme from wordrpress.org thats what they suggest me and still having many issues. The CSS in not responding, even if i erase the full sheet its the page still displays with its css? why? is there any problem with the cookies? pleas I need help. Thx much. It works the first time its getting installed buy then after a computer restart, nothing.