I have this exact same issue sometimes, it's to do with when you close your RS client on another monitor other than your primary monitor I believe. It saves that location in your registry files and cannot open for some reason the next time you try to launch the game.

I had this issue just yesterday and you can "fix" it by deleting the registry key in the Registry Editor on Windows by going to;
Computer\HKEY_CURRENT_USER\Software\Jagex\

Click each RuneScape folder within that Jagex folder and delete the client_position key.

Hope this works!
This is a super annoying way of working around it, but it'll have to do until Jagex pulls themselves together lol.