They could charge you the overdraft fee, but probably won't (unless they are a large bank - smaller, regional banks seems to be more forgiving). If they do charge you a fee, call customer service and explain that there was an unknown charge taken from your account that put it negative, that without that charge, you wouldn't have gone negative, and ask if they can waive the fee and credit that back to your account. If you have a good relationship with your bank (such as not go negative on the account before), they may just waive it.
Call the Bank and clarify what the unknown charge is for and ask if there will be an overdraw fee. Good Luck
Go back through your receipts to make sure that you didn't debit something that you forgot about. If you don't know what it is, then challenge your bank account and tell them that it is not your's.
However, make sure that it is not your debit.
35 bucks seems to be the average these days. The bank may waive the fee if you can prove fraud. GL.