If a client does not show up for the agreed upon session, you have the option of billing the client for the missed session. To do this, you will "confirm" the session on your dashboard after the session time has passed. 

Please remember,  this is a working relationship between two parties, so even if the cancellation occurs outside of the cancellation policy, it is important to try to work it out directly with your client. Many trainers will let the first one "slide" and be stricter with enforcing the policy for all sessions moving forward.