This shows one of the risks of overpayments on your mortgage.
These overpayments save interest in the long term, but this equity is hard / impossible to liquidate in an emergency.
The OP has an agreement to pay x every month. If X is not paid, the mortgage is in arrears. The bank are right to contact you and tell you that you are in arrears on your loan agreement. But, this does not mean that the bank intend taking you to court in the morning, just that something will have to be done about this.
However, the previous repayments should give the OP more leverage to renegotiate with the bank. But, you will have to do this. Don't ignore the issue. Contact the bank with a plan