Started working before effective date, quit after signing contract, now previous manager is refusing to input my hours for the time before effective date, any recourse?

So like the title says, I stupidly started working for a lab at my university about 2 weeks before I received a contract to sign. About a week after signing the contract, I submitted my resignation and asked for all the hours that I worked before signing to be manually inputted so I can get paid for them. However after my resignation, my manager is saying that he is “unaware of any hours I worked before my effective date” and my emails to HR have been useless. Is there anything else I can do to get paid for my hours or is this just a lost cause? Thanks for any help!