Problem loading website

Please bear with me, as I am not great on WordPress, but am trying to diligently learn as I go.

I wanted to download a new theme, but before I did that, WordPress suggested I backup my files and download the latest version, 3.5.1

I tried to do this, and everything got messed up. Then I read that you shouldn’t skip more than 2 upgrades (I was using 3.4.1 before. So, I tried to delete some files and go back to 3.4.2

I downloaded the 3.4.2 file onto my FTP server, but now I get the following error message when I try to go on my site:

Warning: require(./wp-blog-header.php) [function.require]: failed to open stream: No such file or directory in /home/a7847908/public_html/index.php on line 17

Fatal error: require() [function.require]: Failed opening required ‘./wp-blog-header.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/a7847908/public_html/index.php on line 17

Then, I wanted to see what would happen when I try to go onto my site’s /wp-admin area. This is the error message I received there:

Warning: require_once(/home/a7847908/public_html/wp-load.php) [function.require-once]: failed to open stream: No such file or directory in /home/a7847908/public_html/wp-admin/admin.php on line 30

Fatal error: require_once() [function.require]: Failed opening required ‘/home/a7847908/public_html/wp-load.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/a7847908/public_html/wp-admin/admin.php on line 30

Can anyone help explain this to me and how I can fix it? Also, I tried to restore the backup I had before I did any of this, but the file keeps downloading as an sl file instead of an sl.qz file, and therefore my web host server won’t accept it.

I hope I’ve given enough information to get some help? I appreciate it!