I hesitated to post this, hoping someone find what's happening. Maybe it's better to post my story on this too.
I had this problem a while back. Only one query didn't show the list of tables. Other queries in the same database were just fine.
It was killing me and I decided to do something about it.
I opened a query that was OK in design view and selected show tables from the context menu.
While the list of tables were shown, I right clicked the query in problem, and opened it in design view. Right clicked and selected Show Tables. Apparently nothing happens. Then closed the first (good query) leaving the list of tables to be opened. (didn't close the list of tables)
Then closed the query in problem (again leaving the list of tables open)
As the last step I closed the list tables.
Now if I open the query in question, and select Show tables, everything is just fine.
Seriously, I'm not at all convinced it is NOT opening.
I can't talk for the OP, but in my situation, I have a four monitor set up. Even if I spread the window across all four monitors, it didn't show up.
And one more point. If it's out of screen, why only for one query or for one database. The window that shows the list of tables, stays at the same position where you close it last and doesn't change its position per queries.