Is your OS Windows? If so I am wondering if a virus has attacked your X-Plane installation. My OS is Linux and I have not used Windows for years so my help might be wrong. An alternative explanation is the permissions settings have gone wrong for some reason and your user login has no permission for even viewing the X-Plane directory. In Add Remove Programs is it listed if it would get listed in the first place, as I remember some programs installed in Windows are never listed in Add Remove Programs but if it is you could try repairing, uninstalling, reinstalling in that order, to see if it makes any headway. First thing though would be to use a good anti virus program and do a thorough scan.