I suddenly have blue (or black at night) lots instead of the actual buildings build there
This issue is caused by faulty custom content. You can read all about it here: Game Help:Sims 3 Blue Lots Issue
If your town has the issue and you saved your game, the town cannot be saved. Try restoring a backup game.
Since quite a few files got converted into Sims3packs, you will need to check those with Custard before you install them into your game. Find a list here: BAD CC: TXTC Fehler (Blaue Felder/ Blue Lots). There are probably more out there and you will need to find them in your Launcher and uninstall them. It might be possible that you have a fixed version if someone fixed them with the tools available before uploading but it’s better to be safe then sorry for this is a serious issue that can cost you your town.
I’m only listing sims3packs since the package files can easily be found and fixed with Delphy’s Dashboard. Dashboard has the ability to scan *.dbc files but it can only tell you if something faulty is inside, not fix it. If Dashboard finds something, you will need to locate the guilty party in your Launcher and uninstall from the installed content tab.
Saving your family
If you are still able to enter your house, you can try to save your family to the library and move them to a new town. You will loose all relationships with other families this way but it’s better than nothing.
Twallan’ Porter Mod might be able to save more than just your family AND keep the relationships intact, but I’m no expert on the mod, so you will need to try yourself.
Saving your Houses
Any house that has not turned into a blue lot, can still be saved to the library. Houses that are blue lots cannot be saved.