I say she's in breech of contract and you cancel HER and keep her deposit. Send this letter CERTIFIED MAIL with return receipt and wash your hands of her:
I'm writing to notify you that you are in breech of our contract dated ____________. In paragraph XX it states that final payment is due 30 days prior to your scheduled event. That date has passed, I have not received your final payment and repeated efforts to contract you have been unsuccessful. Unfortunately we are past the point where I can execute the cake originally agreed upon. I am hereby terminating our contract and any monies currently held as a retainer are non-refundable, and thereby forfeit by you.
I agree 100%. This is the perfect professional response. I applaud you for having a business contract. I applaud you for not caring about her reasons, they are her business, not yours. You have done your due diligence and the final letter (above) will seal the end of this transaction.
I'm with Barbara Jean. It isn't necessary for the OP to storm in demanding to know the whys and the wherefores. No matter what the circumstances the bride owes the OP a response as to whether she wants the cake or not. If approaching her in person is the only recourse the OP has left I say go for it.
I politely but emphatically disagree that the bride "owes" the OP a response "no matter the circumstances". The bride fulfilled her part of the contract by submitting $300 in a timely manner to hold the date. The bride knew the contract stated that the money would be lost if the remainder was not paid on time. Even very polite inquires at her place of work are vastly inappropriate and very unprofessional. (I think the original use of the word "storm" was perhaps used for emphasis, not actually to denote "storming" into a business.)