Things can crop up unexpectedly at any time which can force either the applicant or the company to cancel an interview. The less notice you give, the more unpleasant writing an email like this can get.
It is therefore important that you give a reason for canceling wherever possible and suggest an alternative date and time for interviewing the candidate. Try and word your message as transparently as possible.
Dear Ms. Doe,
Unfortunately, we must inform you that we are unable to make the interview at [time] on [date]. We have only just found out that [reason, e.g. the head of department will be on sick leave for the next week].
We hope you understand the situation and we would like to rearrange our interview for [date, e.g. the following week], if possible.
Please let us know if that would be of interest to you. Thank you for your understanding.