Hey Shafto - I ran into the same problem in my FS9 installation and after posting it on the FS9 side, I found out what the problem was.
In their all-knowing way, Microsoft automatically updates everyone's computer that has an MS OS every Tuesday. That is, it will automatically update if your settings (Control Panel - Windows Update, in W7 anyway) allow it to happen.
What temporarily sank your sim was an update called KB3086255. This is an anti-piracy update that changes certain things in your OS that greatly interfere with - or stop altogether - your ability to run your sim.
Solution: In Windows 7, go to Control Panel - Windows Update - View update history. Somewhere in that list of updates is the one referenced above. There is a way to run a search for the update by typing it into a search box, however, I just went by the date the event occurred and matched it to the "Date Installed" column on the right, then checked to the left for every entry on that date and found it right away. You then need to follow the instructions to delete the update.
Of course, this will remove whatever anti-piracy protection was in that update, however, it had the vulnerability for years prior to this update, plus good-quality third-party software should offer good protection to you as well.
Once you have removed the update (this will involve restarting the computer after you delete the KB update; if it doesn't tell you to restart it, do it anyway), reopen the Windows Update panel and look at the box at the top of the screen where it tells you what updates are available; that KB update referenced above should be in the "waiting to install" list. Then you know you've removed it.
I did the above and got my FS9 sims back right away. Hope this helps.
