Thanks, but it didn't help. Still Internal Server Error :(

Hi,

I managed to ruin my site. Decided to try JoomSEF, but couldn't make it work properly, so I decided to disable SEF in global configuration and in component and uninstalled the JoomSEF component. First the frontend went down, but at some point backend also. Now the situation is following - with .htaccess file in root directory I get this error message:
Forbidden
You don’t have permission to access /index.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

When I rename .htaccess back to htaccess.txt the error message is following:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I have also Gallery 2 installed in subdomain and this is also down with .htaccess in home directory. With htaccess.txt the gallery is working.

Any ideas, what went wrong and how to get my site up and running again?