If you don't attend a gig you accepted and didn't cancel, the agency can record it as a no-show.
No fee is payable for a performance that didn't happen, and any payment recorded for it is voided. The no-show is kept on the record.
Agencies take this seriously, and rightly: a no-show means a venue paid for an evening that didn't happen and a room full of people who were expecting something. It's the fastest way off a roster.
If something genuinely went wrong, an accident, an emergency, tell the agency as soon as you can. They're far more understanding about a bad night than about silence.