The experiment went awry. The world is gone... sealed away or lost to the void. You just managed to activate a failsafe in time, saving but the smallest portion of the world. But there is still hope - perhaps the world can still be repaired - chunk by chunk.
Chunk By Chunk is a mod that starts the world with just a single chunk (16x16 column of world), but provides the ability to grow the world, adding additional chunks over time. Key to this are World Cores - found hidden at the bottom of each chunk. World Cores can also be used to build a World Forge, which provides the ability to refine raw dirt and stone into more World Cores.
There are two options for how the generated - missing chunks are either empty (making a skyworld), or are replaced with bedrock (sealing them).
From version 1.6.0 onwards:
You can now configure Chunk By Chunk from the main menu, using the Mods... menu prior to creating a world to configure the settings for that world. For fabric or quilt this requires the Mod Menu mod.