Minecraft: Why did modded blocks replace each other throughout the world?

Solution 1:

Loading worlds that were created with a set of different mods as to those you have now almost always leads to some form of corruption; Forge even warns you if you do so. It may be a problem with conflicting item IDs, where the items in Harvestcraft had the same numerical IDs as those in Mr Crayfish's Furniture Mod.

Minecraft doesn't (by default) automatically create backups of your world. You can try loading the world again with the mods you used to create it, however, the corruption is already done, so this likely won't work. As fyrepenguin said, you can type /seed to get the seed and generate the world again, however all your progress will be gone.

In the future, I recommend backing up your worlds and not loading worlds created with different mods.