I ran into this problem, essentially.
You can see where the cables join in the trunk on a moving plate, center of the trunk.
If one side is "locked" you need to get a stiff J-Hook; my mechanic made his out of brake tube, which is steel. Insert it into the space where the trunk hinge is, and you need to fish around to find and then pull the lever to pop the offending side up. Once it's up, and you look inside, the problem should be obvious.
In my case, it was the driver's side that was the issue. The nut on the end of the shaft that hold the lever/cable had fallen off.