DON'T PANIC!
(in large, friendly letters)
I have currently locked the guild bank down (nothing can be withdrawn and nothing can be deposited) because of a bug that happens sometimes when leadership of a guild is transferred from one player to another.
While the items appear to be gone, they are all actually still there. They just aren't visible. They will be visible again on Tuesday when the server resets (this is currently the only fix for this bug, Game Masters and Blizzard technical support can do nothing about it). The only time a problem would occur is if a player deposited an item into a slot that's already filled. If that happened, when the server resets on Tuesday, it would reset with the new item and the original item would be replaced (hence why deposits are removed for the time being). So, we haven't been hacked and access will be restored. We've just been bugged temporarily. Sorry for the inconvenience!