Ive just experienced a similar thing, causing google to prompt my website as spam/phishing and erroring a 403. ive emailed my host and within minutes got this response.
It looks like a vulnerability in your site has been exploited, or else somebody has access to your details. Were you running some open source software (such as Wordpress or Joomla)?
Hacks like this are generally caused for one of two reasons.
1. The most common (in 90% of cases) is that your software has a vulnerability. This could be with the software itself (for instance Wordpress) or with a plugin or template. For instance, lots of Wordpress themes use Timthumb, and an older version of this was vulnerable.
Because open source software code is by its nature available to anyone, vulnerabilities are very quickly disseminated throughout the web. If you run an unpatched version, then it can be exploited, and scriptkiddies search for vulnerable targets.
2. The other cause could be that somebody knows your password. This will either be because it was weak and has been brute forced, or else you have a virus on your computer.
I would suggest the following steps:
First, if you can let me know when your site was last working, I can restore from a backup. This will roll your site back to how it was. That shouldn't take longer than a few minutes, but Google / your browser may show a warning for a short while until it updates.
Second, you should then upgrade all your site software. This includes any plugins and themes - and I'd suggest removing anything you do not need.
Third, as a precaution, you should change your control panel password. While it's unlikely the hackers got this, it's best to be safe.
Finally, I'd suggest running Malwarebytes on your computer. This is a good piece of software that scans your own computer for viruses. You can download it at
http://www.malwarebytes.org/ as there is a free version available.
If you can let me know when to roll the site back to, I'll do that for you.
Kind Regards,
