Hi Asha, I believe that Clarity v14.4 introduced the ability for the Cost Plan of Record to be deleted. This is one of the points noted in the release notes. I remember having arguments with CA Support when this feature was introduced because there is no way to disable this feature. What is even more silly is that if a Budget Plan was approved from the Cost Plan, you can delete the Cost Plan - but the Budget Plan is still there with no frame of reference to how it was created.
My only recommendation is that the Cost Plan is manually recreated based on the current Budget Plan (if that exists). If you have not switched on Audit, then the Cost Plan is effectively lost.