You will need to contact your Landlord, apologise and make a valid reason for missing your payment and confirm to them that the late payment has now been made.
If you are unable to make the payment please contact your Landlord and explain your situation. Landlords like to know of any problems that may arise and in most cases will be very understanding. The worst thing you can do is to fail to make contact with your Landlord as this may result in him commencing legal proceedings against you for the termination of your lease and your eviction.
It is also worth nothing that bad references from Landlords and credit agencies will affect you when seeking accommodation in the future.