1. Secure the owner account
If you're the server owner, change your password right away and turn on two-factor authentication (2FA). Log out sessions you don't recognize in User Settings › Devices, and remove apps you don't recognize from Authorized Apps. If you can't log in at all, contact Discord support.
Accounts are usually stolen with "scan this QR code", "Nitro gift" or "can you try my game" files; warn your staff about these too.