When a change in
.htaccess results in the page being empty this means there was an error parsing the file, most frequently because of typos or referring to a module that's not loaded. You can check the problem in the Apache error log - check the
ErrorLog directive in the vhost config for its location, otherwise it's in the general Apache error log (on Linux usually
If the error is something akin to
Unknown option RewriteEngine you haven't enabled
mod_rewrite correctly. On Debian/Ubuntu systems you can fix this by running
a2enmod rewrite, for other systems the instructions could vary.