Fatal Error after update 1.7.1 to 1.7.2.

This afternoon I tried to update Joomla to the latest version (1.7.2.) with the internal update function, unfortunately, the process have been interrupted. Since then the website is not working anymore and the front-end is showing the following fatal error:

Fatal error: Call to a member function mark() on a non-object in /xxx/xxxxx/xxxxx/public_html/index.php on line 25

At the moment I don't have any backup, the website was in progress but almost finished.
I've already tried to solve the problem, however, without any luck. Who knows how to solve this issue?

Well backup what you have

Download joomla 1.7.2
copy over your website overwriting other files.
Dont upload installation folder.

The solution of NormanUK worked!

All the files on the server, including the template files, were downsized to 0 bytes.
I just uploaded all the files of Joomla and template to the server, excluding the Installation map.
The database was still oke, however, I had to manually reinstall all the modules and plugins again, including Image Show and News Show Pro etc.