I would try the Repair option on FSX that is offered first. If that doesn't work then a uninstall/reinstall by the book installation.
To get the Repair option, a way is to go into the Control Panel, depending on what version Windows you are running locate the Add and Remove programs, and treat as a Remove, the window will open and ask Repair or Uninstall.
I would say another way is to put your FSX disc in and you could also be prompted to do the Repair that way as well.
FSX has been around for a few years, and is a great program, all the advice and forums dealing with FSX keeps this program in a constant revitalization for those that choose to keep up with the latest and greatest discovers. This to me actually keeps the program New.
Muliplayer Support for FSX was not done away with because it was old. That's a whole other story one that I will not comment about here in this forum. I will say that opinions by others had a lot to do with it

There are many FSHost servers out there that accomodate the multiplayer need now, you will need the chocolate client software in most cases to utilize the FSHost servers that were provided by many sites now.