A submatrix collapse was a type of catastrophic failure that a Borg cube could suffer. When a cube experienced a submatrix collapse, the Borg Collective immediately severed its link to the afflicted population, considering it as good as dead. Drones remaining inside the cube may enter a state of regenestasis. (PIC: "Maps and Legends")
In the late-2380s, a Borg cube suffered a submatrix collapse after assimilating Ramdha, a Romulan who had experienced the Admonition, due to the sheer weight of her despair. (PIC: "Broken Pieces")