You have several options here!
I believe there is already a thread @alexs?
However,
If you have PayPal you can withdraw funds from your prepaid card into PayPal then back into your Monzo CA.
If you want, you can take all the money in your prepaid card out in cash at an ATM and pay into another current account the transfer to Monzo.
You can also ask Monzo to refund the balance of your prepaid card back to the original top up card.
Hope that helps