Unless that last year included keeping all of the scripts up to date, I'd guess there's your problem.
Check out all of your .htaccess files, look for new odd files, etc...
Someone could keyword stuff your page (likely with scraped material), but only show it to user-agents like Google. They could also be redirecting certain traffic based on whether or not it's being referred from search engines.
Depending how it's done, more than one site could be affected, especially if they're all under the same user.
In any case, a host can boot you at any time and they all have a TOS that protects them. The only thing a lawyer could do is take some of your money.
I'm also pretty sure that once DH receives a legal threat, they just cut you off & ignore you, so that definitely wouldn't be the best route for getting more info out of them.
I'd also guess at this point, you'll need to deal with abuse instead of support, so the replies aren't likely to be super-speedy as it would be a lower priority.
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