Add own theme/template

Hi
After I started to update the matrix template the index is not working anymore and only a blank site is shown
http://www.imageview.ch/gallery/

http://www.imageview.ch/gallery/main.php is still working as I had a backup and copied the backup files back to the server.

I changed the files in themes/matrix/templates/ as I wane implement my own template of the website.
How is it possible that http://www.imageview.ch/gallery/ will work again and how am I able to add my template into the gallary

Thanks