Here is the FAR on ceilings.

16.301-1 Description.

Cost-reimbursement types of contracts provide for payment of allowable incurred costs, to the extent prescribed in the contract. These contracts establish an estimate of total cost for the purpose of obligating funds and establishing a ceiling that the contractor may not exceed (except at its own risk) without the approval of the contracting officer.

There are two possible solutions.  You could find additional funds and get the contracting officer to increase the ceiling.  The other is if  the Ceiling has a statutory limit of some type,  you would need to reduce scope to stay within the statutory dollar limit.