Last night the game, Sims 2, crashed and I rebooted before trying again. It was slow so I used 'force shutdown' which I've never done before. Now when I try to load the game, nothing happens at all. I know that Sims2EP9.exe is loading (saw it with Task Manager) but then it quits and nothing ever comes up. Any suggestions ? Would it cause problems if I reintalled Mansion and Garden Stuff again, that was the last one installed ?

Don't reinstall; first try deleting your caches and then test the game.

However, it's a bad idea to force shutdown as Sims 2 writes to the neighborhood files upon exit, so it is possible that your hoods have suffered some corruption.