I have tried everything "a serious error has occurred"

irisgaljack​  Do you still have your main save to test with?  Do new saves currently work, after you've done all these various troubleshooting steps?  Please test a new save in Monte Vista if you haven't already.

And to clarify, please list the full file path to a new save you create for testing.  Find the save in Documents > Electronic Arts > The Sims 3 > Saves, right-click it, select Properties > General, and list the Location.

yes new saves work with monte vista and others and yes the save files are located in the correct location of

C:\Users\[me]\Documents\Electronic Arts\The Sims 3

irisgaljack​  Okay, so it's only the main save that won't load?  If so, try a backup of this save.  Even if you never made one, the game keeps the files from the second-to-last time you saved.  Open Documents > Electronic Arts > The Sims 3 > Saves, and you'll see a folder with your save name and another with the same name and a .backup extension.  Rename the backup and remove the .backup extension, so the folder name ends in .sims3.  This save will now be available to load at the Main Menu.

If it loads and you can play it at all, I would keep a spare copy of this backup in case you need it later.

Separately, when working on a significant or time-consuming build, I would recommend doing so in a new save, either in the same world you're using or in a builders' world if you want to install one.  You can save your completed build to the in-game bin and place it in the save where you want to use it.  You can also save partial builds to continue working on later.

Part of why building in a new save is helpful is that a new save will load much more quickly, and a builders' world is extremely quick because there are no sims there.  Build/Buy is more demanding in some ways than Live mode, and you'll notice the game start to lag heavily at times, in particular if you've been in BB for a while or have been using Create a Style.  The only way to fix this is to quit and reload, which you might need to do several times before you complete your build.

The other reason is since BB is so demanding, and in different ways than live mode, a long build session in an existing save (where you have sims you've progressed as well as other data) can overwhelm the game engine, dragging down performance and possibly even corrupting the save if some data gets lost or mangled.  So rather than risk damaging your main save, you can build elsewhere, then just plop down the completed build when you're ready to play with it.

yes iv tried the back up yes i have a copy of both saved and renamed so i can use them to test and no neither of them work

irisgaljack​  If the backup won't load either, but new saves in MV work fine, then it's possible this save is too corrupt to salvage, unfortunately.  But there is one other step you can try.  Create another new save in MV, play in live mode for a few minutes, save your progress, and quit.  Open the folder for your new save and delete the Monte Vista.nhd file inside.  Copy the Monte Vista.nhd file from your old save (use the backup since it's probably less corrupt) into the folder for the new save.

When you get to the Main Menu, your new save will show up, looking as it did when you saved it.  But if it actually loads, you'll see your old save instead.  If this does work, I would strongly recommend saving your households and builds to the in-game bin, as you'll probably need to port everything to a new save anyway.  But hopefully you can rescue what you don't want to lose.

Don't save your progress in this "new" save after you've swapped out the .nhd file.  Instead, if it does load, this would be the time to make sure you have the NRaas mods that help with save corruption issues (ErrorTrap, Overwatch, MasterController, possibly others but only if we see a scripterror that suggests using one) and no other mods or cc at all.  With only these mods in place, run an MC town reset, then "save as" to rename the save, quit, reload, and see whether the save still works.

Every time you quit the game, or it crashes or you have to force-close it, delete the five cache files in your user folder:

• CASPartCache.package
• compositorCache.package
• scriptCache.package
• simCompositorCache.package
• socialCache.package

Additionally, open CurrentGame in the same Sims 3 folder and delete anything inside.  This folder should always be empty unless you currently have a save loaded, but the game doesn't always delete the files inside after a crash.