Objects disappears (stairs only so far)

Before I start I want to apologize in advance, but I think I’m the worst explainer in this universe lol but anyways, here we go:
So I started this new project a while ago and then I created some stairs, everything was fine, and as I went on with my building my stairs started to “disappear”, even though the objects are still placed, and if my characters walks on it he kinda get stuck there, this is really frustrating as my day has been quite bad already lol.
I’ve only placed some platforms and some stairs so I can’t really tell if other objects will act like this as well, this is really annoying and kinda destroys my design.
I haven’t changed any settings for this to occur, please help.