One of your customers did not attend his appointment and did not contact you. You can easily mark appointment as a no-show so you keep track of it in your customer history.
For this:
Go on the Calendar tab
Select the appointment you want to mark as a No-Show!
Right click on it, and select the option No-Show!
The appointment becomes purple, and you're done!
Note: From the customer booking history you can easily check if there has been a lot of No-Shows!
Cancel a No-Show
You can can a appointment market as No-Show at any time.
For this:
Go on the Calendar tab
Select the appointment you want to mark as a No-Show!
Right click on it, and select the option Undo No-Show!
The appointment becomes green again and you're done!
Consult this article to complete an appointment.