If you are editing the .htaccess file in your "www" directory, you have to be careful not to have any typos or syntax errors. One simple typo can cause your entire website to stop working.
Any time you make an edit to a .htaccess file, make sure you test your site afterwards to insure everything is still working.
If you want to test your .htaccess file before making it live, we created a .htaccess checker tool which can be found at:
You can paste or upload your .htaccess file to the above site, and it will look for any syntax errors in your file. It will let you know which lines are problematic, as well as a description of the problem.