If the rent is not paid you must take action. After the rent is past the agreed upon due date, hopefully spelled out in a written rental agreement, it is the onwer or the owner's property manager that must take the next step.
The next step is to call and see what is going on. Whether the news is reasonable or not, I suggest a 3 Day Notice to Pay Rent or Quit be served.
To serve it legally, you must personally deliver the notice to the property and hand it to the resident or a person that can accept substitute service of the notice. You cannot email or fax this notice for it to be usable in court. After an attempt, you can post and mail the notice as well.