Back in May I decided to reload my Purchased Sims 3 onto my PC to show a friend the Ghost Hunter and the Witches since she never seen the game. I placed the game in the town Witchwater and created my Sim , and her Sim as witches , I also created a Cat and Dog. and joined them as family.

I Bought the house we moved in and got everything setup. I assigned her the Ghost Hunter Profession made sure everything was working and saved the game back on June 7th. Total play time was 6 hours.

Today when she came over I loaded the game to show her.. When it finally loaded to the house the only thing that was available was live mode. Buy and Build were not clickable. No other characters were shown on the left side to switch to.
If I zoomed into the house the walls would go down and you see grass , No Sims or No Furniture other than ZZZZ showing where the characters were sleeping as where I left them when I saved the game last.

I quit the game and relaunched it several times it does the same thing.

But I can go into my Test Saved game with no issues and also create a new game with no issues. But the Witchwater saved game is just not working and not sure how to fix it or what happened.

Thanks

I'm not sure what would cause that, but it sounds like the save has been corrupted and may not be a way to fix it. 
In the future, it's good practice to do Save As rather than just save so if something does go wrong, you know you have a previous save that works and hopefully without too much progress lost. 
Though you could check in your Saves folder for a .backup save folder and see if that works. 

As for stopping it happen again, maybe others have some idea of what causes this, but there are of course the usual things like checking your mods are up to date/conflicts or corrupted things