Help! Fatal error: Can`t log into Admin

Hello, I have just started using zen cart, and i like it a lot.
I did not log of my server yesterday, but when I refreshed my web browser, I got this error when I starts my admin log in site.

Warning: require(includes/application_top.php) [function.require]: failed to open stream: No such file or directory in /home/x/x/xxx/www/xxxxxxxxx/admin/index.php on line 23

Fatal error: require() [function.require]: Failed opening required 'includes/application_top.php' (include_path='.:/usr/local/lib/php') in /home/x/x/xxx/www/xxxxxxxxx/admin/index.php on line 23

Help! I`m a novice in zen cart, but I thought I was starting to master this.. :(

But the webstore works, but not the admin part....

Should I have admin/includes folder?
It seems like its gone here......

Yes, you should have an /admin/includes folder.
If you're missing the folder, perhaps you dragged it into another folder by mistake?

Probably I have deleted it, or something is weird....

I have transfered the admin/includes once more, but I have to install it once more.

What shall I do?

I think I have to backup the files that are uniqe for me, and install all once more.

What files shall I backup so I don`t loose all my changes/database and so?

Or can I just start the installer once more, and my stylesheets/database will not be touched?