Cant get into wp-admin

Hi,

Can not get into my wp-admin. Everything was fine a few days ago, the site is still running fine without problem but when I try to get into the wp-admin to make some page changes I am receiving the following error…

Parse error: syntax error, unexpected end of file in /home//public_html/sitename/wp-admin/includes/theme.php on line 239

I have opened this file and can see that it ends at line 239.. no idea why, but I have downloaded and checked how many lines should be in that file and it seems to be well over 1000. So, for some reason the theme.php file has deleted a load of lines from the code.

How do I restore this file? It was a fantastico install from my host.