I would not pay for services I did not receive unless I was paying say, monthly installments for a set number of sessions. I mean, people do it with trainers and some other professionals, but I would hesitate to enter into any contract of the sort that did not have a caveat allowing for make-up sessions. If I pay for 30 sessions, I want 30 sessions even if I'm unavailable one week. I think the only exception I would tolerate would be for a group class (if I skipped a class in college, I wasn't prorated for the number I actually attended)...
definitely check out if that's a misunderstanding or error in billing, and potentially have a conversation with your t...