If you don't see any buildings, the problem doesn't have anything to do with your activation. If the scenery requires a reactivation, it will NOT disappear, it will just revert in Trial mode, which means it will work in full, but only for 5-6 minutes per sessions.
If your scenery disappeared, it means something in your system has blocked the software from running. The most common case it's the antivirus so:
- Configure the antivirus to exclude the whole Addon Manager folder from scanning.
- Reinstall the scenery using only the latest installer on the site.