How do I get the detent unstuck?

Apply heat to the edges and lightly tap around the detent with a small hammer. Do not smash the detent directly.

The heat and vibration may get things moving again.

Another method:

Apply a couple of drops of Kroil to the pin and let soak over night to penetrate.
Then use a plastic screwdriver handle to rap around the sight base.
Usually the pin will pop up.

If this fails, drive out the front sight base pin and use a small screwdriver or pick to lever the pin and spring out.

Clean out the hole with a small brush or whatever you have, clean the pin and spring and if necessary, sand the pin diameter down slightly.  Lube up and reinstall.

I feel your pain. I've soaked and soaked with PB Blaster, rapped and tapped, pushed the front pin out (what a PITA) and still can't free up mine. I'm contemplating drilling it out but it's a Yugo so it's a different size and no spare replacements easily found.

If you do need to drill it out, CNCwarrior has replacements.

Thanks for the responses. I let it soak in Hoppes just to see if that worked and it didnt. I read elsewhere to use WD40. I will probably just tap out the front sight pin and try to work it out myself. If not, I will take it to a gunsmith and have them fix it for me. lol