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