Depending on the size and activity, it was probably "cleaned up". Do you happen to remember the URL for the site? You can check to see if a dump is available. This would at least let you get a copy of all the pages (files not included unfortunately). Either way, if you want to know why, request that it be restored, or have any other questions, it would be best to contact staff using Special:Contact/general.
Hi, Andrew is correct- and the link he gave you to contact staff is where you contact staff. When you went to the URL, does it said "Invalid Community" or "The wiki is closed"? If the wiki is closed, then that's where you contact staff to restore it for you.