They did the same thing a couple years ago (or last year? cant remember), but they actually posted a message in the website explaining why they were shutting down. They eventually came back a couple weeks later.
I'd suggest you to follow them on twitter to find out whats going on:
http://twitter.com/AlizeeCommunity
Looks like they're coming back with a new forum that is going to be called AlizeeCommunity, I think Sebastien is not gonna be running the site anymore, only Julien et des autres francaises. Why are they shutting down ol good AFC? That I don't know, but I guess its just them not being happy with Alizee's attitude towards her career. Who knows, I haven't read their forum in ages.