You should notify your clients as soon as is practical, and cancel/rebook the session on your therapist app. Such instances are, in our experience, rare. If they occur on a frequent basis it will reflect in your quality score as well as risk your profile being dropped from display.
If you are struggling with your health or personal circumstances, note that you do have the option to hide your profile during months it's not possible for you to maintain your usual standards of client care.