undefined function error – version 2.1

We have version 2.1 of WordPress installed. My daughter made a change to the index.php – just trying to install a watermark (background) image that wouldn’t work so she deleted just that line again. But now she’s getting this message:

Fatal error: Call to undefined function: get_header() in /home/thomases/public_html/equus/index.php on line 1

We tried reuploading a fresh copy of index.php for the default theme but it is still showing that message. What else can we do to fix this? We tried renaming default theme to default2 and then uploaded a fresh untouched copy of default and resetting that as the presentation theme but it still shows that message at https://equus.thomases.org/ so we’re stumped. Can anyone advise please? Thanks in advance.