Be sure you use the latest vehicles update: they might be there, but floating in the air, because you have applied a recent Update without paying attentions to the release notes that in December required for a full GSX installation using the current full installer, or a download of the vehicles update installer.
If they are not flying in the air, then your problem is different, and it might be related to a wrong video tweak that you shouldn't have used. This is usually fixed by removing the %APPDATA%\Microsoft\FSX\FSX.CFG file, in order to have the sim create a new one with all default settings.