http://www.cic.gc.ca/english/information/faq/citizenship/index.asp said:
What happens if I miss my scheduled citizenship test, my interview with a citizenship judge or my scheduled ceremony?
We will mail you letters telling you when and where to go for your citizenship test and for your citizenship ceremony.
If you do not appear for your scheduled citizenship test, we will mail you a letter telling you that because you missed your citizenship test, you are now scheduled to appear for an interview with a citizenship judge.
If you miss that interview, another interview will be scheduled for you and you will receive a notice of that second scheduled date by registered mail.
If you do not attend the second scheduled interview, your file will be closed.
If you do not attend the citizenship ceremony and do not contact the citizenship office within 60 days, your file will be closed. You will then have to fill out another application form and pay the fee to go through the entire process again.