Change in Fixed Price Deliverables

Hi all, would love some thoughts on this scenario. 

We have a fixed-price contract that was proposed based on the completion of 50 IT assessments, which came to about $50K per assessment, for a total of $2.5M for the year. In the proposal and the kick off meeting, the Contractor proposed their schedule to account for all assessment. Part of the SOW also included the Contractor to be in charge for providing a schedule management plan.

Fast forward to 3 months after award, and 7 of 10 assessments planned so far have been cancelled by a Government POC (not COR nor CO), sometimes within 3 days to an assessment. Due to the invoicing clause in the contract, which states that invoicing cannot be done until an assessment is complete, the Contractor has not been able to invoice for any of non-completed assessments, even though they have incurred some charges.

As of now the Government plans to move all cancelled assessments until later in the year, although a specific timeline has not been given.

The Contractor is starting to have staffing issues due to the lack of work, and lack of payment. 

What is the Government versus Contractor responsible for? What should we be looking out for in this scenario? Since no specific schedule was included in the contract nor its attachment, does the Government have the right to change them all?