Guys used to bring me Smiths and Colts that they had lubed with WD40 for years. They would either be gummed up so bad they wouldn't fall hard enough to bust the primer, or were as in your case locked up from being stored. I disassembled them and then cleaned them up.
In your case not to damage screw heads and devalue the piece, take the grips off, and soak it in mineral spirits for several days. By soak I mean submerge like put it in a bowl, or cake small cake pan, and pour in enough mineral spirits to cover the pistol.
You can leave in that for as long as it takes to dissolve what ever is causing the lockup, provided it is a grease problem. When the gun frees up, you can blow it out with an air hose, wash it out with carburetor cleaner, blow it out again, and IMMEDIATELY OIL.