What do you mean by "stability"?

If you were to apply an in-plane load on a section of one of the free edges, there is a critical load when the flat plate will buckle out of plane. Is that what you mean by "stability"?  If so, you need to apply that load in a Static Structural analysis and then link the Solution cell of that to the Setup cell of an Eigenvalue Buckling analysis to find out the Critical Load Factor.