Just a guess here
...
If you can protect the finish of the gun, and put it tight in a vice, you might try a piece of 2 by 4 wood, say 12" long, and put end on slide. while holding it, and give it a good whack or 3, with a hammer, hopefully the wood will force the slide back, and not damage the slide in any way.
Make sure magazine (owners manual) is out or in, as some guns have no slide release, other than pulling back the slide (racking it) once the gun fires its last round, the slide stays open until another mag is inserted with, or without, ammo, and then the manual racking of the slide to return it..
Does the barrel tilt up for cambering? Or does the barrel stay straight? Is there any free movement of the slide, pulling back on it?
Luck,
LS
PS.. may have read this wrong, is the slide release lever not letting the slide return, and it's the return lever that seems stuck, holding the slide open?
If so, same deal, vice, wood, hammer, whack down, on slide release lever, and hopefully. If this works, then inspect lever for movement, lubricate if needed, but if it remains stiff, not free, easy movement, then off to the gunsmith.
Otherwise, bright light, magazine out, look up on the left side and you should be able to see slide release area,
PSS.. one last "desperate approach".. when I used to build Ultralights aircraft, on one make and model, you, I had to put the front wheel rod, in the freezer over night, to shrink it, so it would go thru. Cold will shrink metal, and may release the metal stress holding whatever it is. Again, all just a guess.