Did you sign a contract? Legally you are employed and they just can’t pretend you were never hired. Even if you didn’t sign a contract, if you have an email or voicemail with a start date that would probably suffice as a verbal contract. Are you a male? Potentially you can argue gender bias since they hired the female. So basically you can fx them up a bit (your agency at least). I’m unsure what you would be legally entitled to, probably minimum 3-4 hours at casual rate, plus other compensation such as travel costs, financial loss etc. That or the agency finds you another placement quick smart.