Unfortunately there currently is no process for reversing cancellations.
If there is correspondence or reports referring to this grant number, you can restore the grant amount by doing a grant amount correction. This will not remove the cancellation, but it will result in the grant showing up in reports with the correct amount. You can add a note regarding the erroneous cancellation to the file or application.
If you require a clean grant, you would re-enter the grant and make reference to the original one.