Answer:
Prometaphase
Step-by-step explanation:
During prophase, chromatin condenses, so the chromosomes become visible, and once this is completed, it enters prometaphase (right before entering metaphase), where the nuclear envelope starts breaking down to allow the chromosomes to migrate to the equator of the cell, which occurs during metaphase. If the nuclear envelope doesn't break down, the cell can't continue into metaphase, so it is arrested at prometaphase.