If the raptorial appendages become disabled and a stomatopod is not molting, they break off the appendages so that they can grow new ones. They also do this sometimes if the appendage is stuck in a net or other object. Molting is the other time when appendages frequently get stuck and are torn off.
This is going to take three or four molts for them to regenerate to full size - probably close to a year. If the animal gets lots of soft food to eat rich in calcium, it may well survive.
Why this happens is generally a mystery, although many kinds of stress can cause it. These include cold, heat, salinity, organic solvents in the water, low oxygen, heavy metals, etc. The metabolic pathway that powers the strike is based on a very high energy, but sensitive arginine phosphate system. When something destroys the muscle, the animal appears to have no choice but to tear off the appendages and start over.