When you clear an interview in an organization and are offered a job, after mutual discussion a date is agreed upon on which you would be joining the organization. That date is specified in the Offer Letter as well. Sometimes due to exigencies, it becomes impossible to join on the said date. Exigencies can be any - a sudden death in a family; you meet with an accident; you or any other member in a family severely ill etc.
So in that situation, you can write an e-mail to the person concerned in the HR department for extending the date of joining. See the example given below.
Hope you are doing good. I would like to bring to your notice that I met with an accident and got a minor fracture in my right hand. Due to which I will not be able to join IBM on August 1, 2010 as mutually agreed upon earlier.
I would request you to extend my date of joining from August 1 to August 15.
I will be grateful to you if you'll consider my request. Thanks for understanding my situation.
With warm regards,
(write your name)