As explained, many times already, if you just don't see any buildings, it doesn't have anything to do with activation, or the loss of it.
If you lose your activation, the scenery will not just disappear: it will revert to Trial mode, which means it will run in full, just for 5 minutes at a time, then you'll restart FS9, and get another 5 minutes, and so on.
Also, since you are seeing the Addon Manager menu, which confirms your scenery is Active (and, as I've said, if it wasn't, you would see *all* buildings, for 5 minutes at time), this excludes the possible issue of not having the Addon Manager running, usually because the antivirus, which is the only thing that would give you no buildings, not even for 5 minutes.
Since you haven't revert into Trial mode, and you see the Addon Manager, this means everything is working, and the loss of the scenery is caused by something that doens't have anything to do with our software.
First, check you have the scenery visible in the Scenery Library.
Also, there was an issue with a freeware scenery.cfg editor, that changed the case of the [Area] commands in the scenery.cfg from the standard that FS9 uses (which is with Upper case A ), to [area] (with lower case a), this confused the Addon Manager, that needs to scan the scenery.cfg, in order to disable Trial messages, if you fly in an area you don't have the scenery for.
This has been fixed in recent Addon Manager releases so, you can try to install the latest Stand-Alone Addon Manager for FS9, available on each product "Info" page.