Assuming Explorer Replacement is turned on, if you double-click a folder on the desktop, does Opus open then?
Or if you launch Opus a second time, does it open?
If you look in Task Manager, is there a dopus.exe?
Note that if you run dopus.exe elevated (as Administrator), most of the desktop will not be able to talk to it to ask it to open a new window. So if it is already running elevated in the background, that may be the cause of the problem. More detail on that here: Why NOT to run Opus as Administrator under UAC