Well there are two things that culd become of this:
After checking IPB's admin interface, there is no option that I see to wipe any mysql databases clean. Meaning most of the posts, users, and such should be saved.
But since the hacker got into the controls enough to turn the board offline, then most likely he would delete the admins of the server, giving them no access to the forum to turn the forum back on. Ultimetly, they would have to re-install the forums to get them to turn back on, or change a certain setting in the mysql database.
All in all it still sucks. And the guy who hacked it needs to learn some grammer. =\