If you have a broken save (greyed out/can't open) or saying something about can't load this save because it's saved with a pack or town that's not installed.
For this example, I'm using Riverview.
Take a look at the specific save file to see if it has it's metadata file.
\Documents\Electronic Arts\The Sims 3\Saves\(Your save file)
If it's missing it's meta.data file, do the following:
1. Load a new copy of the town you're playing in (Riverview) and save it temporarily. This creates the meta.data file you need.
2. Exit the game, go to the new RV save file and copy and paste the meta.data file to your broken RV save.
3. Load the game to see that the broken save has been fixed and can be loaded.
-- While testing I have found taking a copy of the meta.data file from Riverview, for example, won't help fix a broken Sunset Valley save. To fix Sunset Valley, you'll need to load a new copy of Sunset Valley to fix it.
Note: This post has been locked as it's for informational purposes.