Stair treads/risers separated, no access to underside

Does anyone have advice on how to pull several stair risers back into contact with the treads where they have separated? I just bought a 1920's house and a few of the risers appear to have been "kicked inward" for lack of a better descripton. There is NO access to the underside of the staircase. The area underneath is finished plaster and lathe. The stair treads feel solid, but the 1/2 inch gap is unsightly. This would likely be a DIY project so any descriptions would be gladly welcomed! (attached pic does not show gaps, just for reference of enclosed staircase)
q stair treads risers separated no access to underside, home maintenance repairs, how to, stairs
 19 answers