A client terminates the contract without fulfilling the obligations. What to do?

Dear sir/madam,

I have a contract with a client who has terminated it but they do not want to pay for either the fee for the delay in the first payment (which is included in the contract) nor for the 1 month which is pending due to the 30 days termination period stated in the contract.

Kindly advise on my rights regarding both parts.

Thanks!