Sallie Mae Refund Check Question?

Hello everyone. I found that I had refund checks that I just found worth a quite of bit of money. However, These checks are from months ago and our now voided. What do I do now. Should I go back to my school and ask to have the checks reissued or is it too late and I can't ever get this money? I never knew I was entitled to this money and just found out and I just now discovered these checks Is there anything I could possibly do to get this money back? Thanks to anyone who answers.