I would imagine that the treads,(what your feet hit as you accend) have worked loose and are actually rubbing the nails that were intended to hold them down.
The nail has worked its way out some. the tread is going up and down on this nail causing a squeak. If you found the nail, you could lub that nail and stop the squeak, but that is impossible.
What you need to do is to screw down the treads to the tread supports, or go under the stair way, if possible and add shims between the bottom of the tread and the top of the tread supports. This will bandaid the problem.
This is a good two person task, have a friend stand on each individual tread while you determine if that one squeaks and do the repairs from below.