Isohunt had become a target for the MPAA approximately a year ago when they filed a lawsuit against the company that alleged massive accounts of copyright infringement. There is some speculation that this shutdown is a direct result of that legal action, but the MPAA has yet to announce victory on their website, as they're usually very quick to do.
In a statement on their website, Isohunt administrators posted this "Lawyers from our primary ISP decided to pull our plug without any advance notice, as of 14:45 PST. No doubt related to our lawsuit brought by the MPAA, but we don't have more information at this time until people responsible comes to work tomorrow. We will be back in operation once we sort out this mess with our current ISP, or we get new hardware ready at our new ISP."
Source:
Isohunt.com