Forgive me for the obvious, but is the safety on? The safety blocks the slide.
If that is not the case, my concern is that the gun might be loaded, so the first thing I would do is to insert a rod into the end of the barrel and measure to see if it goes all the way to the rear. If it is an inch or so short, you probably have a round in the chamber, and if the gun is old, the round could be corroded into the chamber. If the rod is around 1/8 inch short of the rear of the barrel, you could have a fired case in the chamber
If there is no round in the chamber, then get a piece of aluminum (preferred) or brass rod that will go into the barrel, insert it, and tap on it with a hammer. (Note, "tap" not pound!) If the slide can move at all, that should move it.
If there is a round, or suspected round in the chamber, and the gun is cocked, find a safe place and try to fire the round. That is the safest way to dispose of it. If the gun is not cocked, then I think I would use the rod the same as above, but even more gently, tapping with something long enough to keep your hands well away from the muzzle.
Jim