First of all you will owe a late payment which is normally 5% of the over due payment.  Second...you will have a late payment (1x30 on your mortgage) reported to all three of the credit bureaus which will bring your credit score down by maytimes up to 50-100 points which will make it hard for you to be able to re-finance your mortgage in the next 12 months.  If there is some sort of hardship that has happened and that is why you are going to be making your mortgage payment late some lenders will consider re-financing your mortgage before the 12 months of on-time payments has passed.

Additionally, you will have to at one point make a double payment (so to speak) to get caught up. Otherwise, you will be considered 30 days late every month thereafter until caught up.