My therapist requires 24 hour notice or you have to pay for the session you missed. I'm not sure how strongly she enforces that if you've had a true emergency and can't go to the session.

I did once lock my keys in my car and couldn't make it to the session. But, I was able to reschedule for the same week, so she didn't charge me.

My therapist seems fairly reasonable and I've never missed a session with her except for the one mentioned above. So, I would hope that if I did suddenly have to miss one, she'd be understanding and not charge.

My psychiatrist has the same 24 hour policy, but I think he's probably more strict about enforcing it.