When in two column mode for alphabetical index the page numbers disappear.
A lot of information is missing: are you in multi-column page context (you page is already 2-column)? is your index 2-column in a single-column page? have you fiddled with the index layout (notably the tab stop for the page number)?
While at it, LO version and OS name.
Provide this information by editing your question, not using your answer since this is not a solution to the problem.
I can’t find tab stops for Index style.