The info is sketchy, so here are some questions:
Previous to this, with no change in pistol, mags or ammo, did the pistol lock on the last round? I'm just looking for a benchmark here.
When you tried different mags, did you fire them, or just use empty mags and cycle the slide manually?
If none of the mags work with manual cycling,
And it worked fine the last time you shot it, something's happened to the slide stop, or,
If the pistol is new (to you), you probably got it with a bad slide stop.
BTW, I'm assuming your pistol is a .45acp Commander. If it's a 38Super or 9mm, the wrong slide stop may have been installed. Within spec, slide stops for different calibers will NOT interchange.
There are many other possibilities at this point, just difficult to work with info currently available.