First thing to do would be to restart D2 in case its just a visual issue. If that doesn't fix it, check this page: https://www.bungie.net/7/en/User/Account/SilverBalanceHistory to see when the negative balance occurred. Theres a chance that you may have bought silver years ago but your platform/bank refunded the transaction which in turn gets the silver removed from your account. If the silver balance page doesn't show it, fill in a missing silver ticket via the 'submit request' button at the bottom of this page and Bungie will look into it. https://help.bungie.net/hc/en-us/articles/360049497031-Contact-Form-Missing-Silver