I am more familiar with Monte Carlos (87), Firebirds (86 and up) and Camaros of this era but this is what I would check since the doors are similar on the inside.
Try the simplest thing first if you have not already, unlock it using the key and the pull at the top. It is possible one of those two is not working and making you think that it is unlocked when it is not. If that does not work unscrew the knob on the interior lock and put the shaft up a bit just incase the linkage is a little out of alignment. This is probably not it but takes five minutes to check.
Next does the exterior handle have any resistance to it when you open it? The linkage may have detached from it. Same with the interior handle. If the linkage has detached attempting to hook the linkage with a bar but that may not be possible.
You may have to find some way to pull back part of the door card, not an easy task on these cars with a closed door. They hook up over the top in the window channel and there is a pair of hooks mid way up on some of them. You may want to Google for an interior diagram of the door. Be real carful of the plastics in the door card.
Hope this helps.
