Script automatically generated in my <head> tag [closed]

Question

I'm sorry, but this script isn't being automatically generated. You have been hacked, my friend.

This happened to one of my test web host accounts once, and with a bit of google-fu I learned that it's a fairly common result of the account having been hacked or otherwise compromised.

In my case, it was ads. In your case, it sounds like whomever got access to your site is using it to distribute some nasty stuff.

You should consult your web hosting company about what you should do and what they can do to help, but at the very least you should:

Take your site down immediately. Stop serving as a malware distribution hub. You're almost certainly losing more visitors by being on the naughty list than you are by having the site return 404s for a while.
Find the source of the intrusion and fix it so it doesn't happen again. Your most likely attack vector is a script/module/other component that bears a known exploit. Your web host may be able to help you here.
Change all of your account passwords associated with that website, including any shell, ftp, database, email, and anything else you have set up. Assume these passwords are in someone's list now.
For that matter, start using a password manager if you aren't already; that's probably what got you in trouble. Sharing passwords between sites is a good way to get owned.
Start looking for a backup that doesn't have the malicious script tag additions and restore to it.
Follow the links in the malware warnings to get your site off the lists once you've fixed the problem.

Good luck!