What do you do when there are some problems with delivering a product/ Service to the customer due to company's faults?

First apologize and then give a next probable date of  delivery which should be at a convenient time to the customer. 

before requesting excuses first we should clear the fault and then while we are delivery the good service again, we can informed to the customer what we have done for them to giving good service by clearing the fault.

In addition to mr. Pathiyil's answer, I will arrange something special to compensate the customer (Gift, additional product,...)