First, be sure you have the P3D V4 Hotfix installed because, if you don't, there was a bug that, if the dll/exe.xml were corrupted by another addon (we don't use them anymore), the sim won't load any other plugin. The solution in this case, would be removing the corrupted dll/exe xml file, or install the hotfix.
If you already have the hotfix installed, be sure you install with the antivirus disabled, and configured to exclude the Addon Manager folder from scanning.