I do not understand how to solve missing values for a prisoner's dilemma matrix.

Im not sure what you mean by "missing values". Is it the case you given a matrix of possible outcomes, where some of the cells are missing and that you are told that a particular outcome in a non-missing cell is a stable (equilibrium) outcome, and you need to infer what must have been the outcomes of missing cells?

btw, its been many moons since I have studied Nash Equilibriums and prisoner dilemma games.