I had that happen when changing steering wheel hubs. The shaft slipped down inside the main shaft and I couldn’t see it to retrieve it.
I took a magnetic extension, the kind used to pick up parts, and ground down the outer casing where the magnet is located until it fit inside the main shaft. With some patience and perserverance I was able to pull the shaft up to where I could grab the end.
I then put a nut on the end so it would not slip back down. Though the hub does not fit over the nut on the shaft the one for the steering wheel itself does.
Since then I have been very careful to either place a nut on the shaft or tighten it so the shaft is as locked in to position as I can get it. I still am very careful to not knock the shaft.
Been there, done that