Does the folder
C:\Users\Steven\AppData\Roaming\McNeel\Rhinoceros\5.0\UI exist? Does it have a
default.rui file in it?
If so, the file may be corrupted. You can first try typing the command _ToolbarReset in Rhino, follow the procedure and see if that fixes it.
If not, in the same folder with
default.rui, there should be a file called
default.rui.rui_bak. With Rhino closed, remove the corrupted default.rui , make a copy of
default.rui.rui_bak in the same folder and rename it to
default.rui. Then restart Rhino.
If neither of those helps, I don’t know what’s gone wrong…
Another thing to try is