That seems odd. Which prepaid tuition plan did you use?
One state has had a tuition freeze in place for a long time. So, in that state, it might be possible for contributions to exceed the cost of tuition.
I'd suggest first contacting the prepaid tuition plan to ask about this. Ask them to refund the excess of contributions over tuition.
One option would be to rollover the funds to a 529 college savings plan or get a refund, since most refund policies will return at least the amount you paid.