How can you reattach a broken reference in a derived split part?
Edit the stock feature at the start of the FeatureManager design tree, then adjust the Defining Stock Part option.
Right-click the filename at the top of the FeatureManager design tree and select Relink Stock Part.
Locate the child part using SOLIDWORKS Explorer and select Relink Parent.
Once derived part references are broken, they cannot be readded. Consider resplitting the part.