Why did my base split into several separate buildings?
Your base is treated as a single building only when all parts are fully connected. Connection means side-to-side contact between structural blocks - for example, a foundation flush against another foundation. Small touches, corners, or overlaps do NOT count as a connection.
If a structural element is removed or destroyed and the remaining pieces are no longer fully connected, the system automatically splits the structure into separate buildings.
What changes after a split:
The Tool Cupboard (TC) stays in only one of the resulting buildings
The other parts no longer have a TC - they begin to decay
You can place a new TC on those parts (subject to building privilege rules)
Items inside (chests, workbenches, etc.) try to power from the TC of whichever part they're in
→ See also: Why can't I place a Tool Cupboard? · How do I merge two buildings into one?