Site's disappeared

Hi,

I had a site up and running fine.

I just went in to settings and was trying to change the index page to a specific page which I’d called home.

After updating the site has not disappeared but it’s lost all reference to the template and styling it’s based on.

More urgently, I can’t get to wp_admin.

I’ve tried deleting and uploading via ftp the index.php but whereas before I was getting page cannot be found, I’m getting this message:
“Fatal error: Call to undefined function get_header() in /home8/allabpd9/public_html/wp-admin/index.php on line 2”

Any advice please. I know it’s something I’ve screwed up but how do I get back in to fix it!!!????

Thanks.