Help me with this strange situation!

Hello,
The problem I am encountering is quite strange. About a month now I own a two-storey building, purchased, but altered in various ways by me. The floor of the upper storey is a sculpted rectangular prim that leaves an empty space for a sculpted 1-prim staircase, which leads to the basement.
Yesterday I realized, not without some surprise, that there is "something" invisible that forbids me from using the staircase. When I attempt to go up to the storey or down to the basement this "something" blocks my way. It seems that this is not exactly at the floor level, but a little bit below it (about one or two centimeters). I checked carefully, and there is neither an actual invisible prim, nor any other known physical object.
I would greatly appreciate if someone could give me any advice on the situation.
Thanks in advance.

It may be a ghost prim, which means your sim needs restarting.

or an actual ghost.............

One guess:
Some sculpted staircases are phantom as their bounding boxes are bigger than the stairs and don't allow it to be used; these are the "for looks" part of a staircase in that situation, the actual non-phantom part being less pretty, solid but invisible prims. Have you turned the staircase solid recently?

Sorry, this is beyond my knowledge. What is a "ghost" prim? And what I can do, considering that I own just a small piece of land and not a whole sim.

It could be that you've somehow removed the phantom setting from the staircase. Often, sculpty staircases need to be set to phantom and have a flat prim rezzed over them, otherwise their bounding box will act as a solid prim and block the way.

Edit the staircase and check the 'stretch' option. If the selection lines surround the staircase and hug it closely, then this is *not* the case, and I agree with Cristalle that a sim restart might be needed. If, however, you see the usual white box around the staircase, with the coloured resizing squares, then try setting the prim to phantom again.

ETA: Osprey types faster than I do, clearly

I'm a left-hand-only hunt and peck typist, and not lefthanded, so if I'm faster than anyone I'm going to give myself a medal

The problem may occur if she linked the phantom objects to the non-phantom objects.

Ghost prims usually occur when something you rezzed and deleted hasn't really deleted - it is not visible in the viewer but it has a physical presence. Usually, you will see the object as being selected/sat upon when nothing is being selected or sat upon in About Land > Objects.

Skell and Osprey and the others have good advice, but maybe I can clarify a little.

There are two possibilities. One is that you have a "ghost prim". This is an artifact, a bug. It seems there is something there, but you can't see it or select it. Restarting the sim will get rid of ghost prims. If you're on a private estate, contact the estate owner. If you are on mainland, file a support ticket with LL.

The second possibility is that your staircase has become non-phantom.
Scultpy stairs consist of two prims...the sculpty, which you see, and an invisible ramp. They cannot be linked into a single object, because the sculpty must remain phantom, while the invisible ramp must remain solid, so you can walk on it.

Sculpties are phantom because they are really spheres. The sculpt map distorts their appearance in your viewer to look like other objects...but physically, they remain spheres, often much larger than the shape that you see on your screen. You can see this sometimes when you first rez a sculpty...it appears as a big ball for a few moments before coalescing into its usual shape. If the phantom box for the sculpty prim is un-checked, you can collide with this unseen sphere.

Well... I just managed to solve the problem!
Mentioning ghosts you reminded me that the visible part of the staircase was originally set to "phantom". This had been altered, probably by a mistake, during some relocation or other editing. I set it back to phantom and now all are OK!
Thank you very much to all of you who responded ot my little problem.
Be well

When you create a sculpty prim and before any changes are made, you get a ball. This is the default shape and size. Place it somewhere and it can be sat on or stood on like and other prim. It might as well be a plywood box or the sphere in how it acts.

Once you put the shape texture on it, the sculpty warps to fit how the texture map directs it. However, you still sit, stand, walk around and all that as if nothing had changed. The sculpty is an illusion in a way. The physical properties never change.

A typical sculpted staircase will consist of two prims. One is the sculpted prim you see that is then set to phantom. The other is a long, flat prim like a ramp that is set to full transparent. What it sounds like is that the sculpted prim is no longer phantom.

You can edit the prim and set it back to phantom with the edit tools. It's really very easy to do.

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Doh!
I took too long to write my reply

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That exactly was the problem. Thanks for the detailed clarification.