Well, as I've said, it's not possible the Addon Manager would disappear without crashing the sim. So, maybe it has simply being blocked by the antivirus, so it cannot "talk" with FSX to issue the command to create its own menu. And for the same reason, the scenery will not show.
Yes, you said you don't have any antivirus but, are you sure you don't have at least the default Windows Defender ?