Guild Bank Hacked
Unfortunately, Anet does not restore guild banks. They even made a post about this already. It would ruin the economy of the game if they did. You can find the post about it in the forum even. It sucks for your guild, but the reason behind it is very sound.
it was someone that left your guild, look to see who left it, and add them to either blocked or friends list, and if the unknown name shows up as that person, you found your thief.
Or it was a hacked account, the hacker who then removed items from the guild bank. Either way, Anet doesn’t restore Guild Bank thievery.
Either way, you cant redeem your items, becareful how you setup your ranks next time.
Lead Game Master
As others have stated, one of the other Lead GMs made a post regarding guild bank restorations. You can find it here: https://forum-en.gw2archive.eu/forum/support/account/Guild-banks-and-Customer-Support/first#post4419815