As soon as you know you will not be able to make the full or any part of the rent, you need to contact your Landlord / Agent immediately. If this is the first instance, the landlord will take a view on how to proceed, however if this is a repeat occurrence, the landlord may opt to serve notice. 

In any instance, you need to keep the landlord / agent fully aware of any difficulties you are having so they can take this into account when trying to decide on the next steps to take.