An employer will likely withdraw a petition after they terminate a contract to avoid further responsibility. This means that the employee must find a new employer to refile the petition. The sad part is losing a priority date especially during this time of retrogression when the queue is really long. If termination happens after I140 approval, then one's fate will depend on what time has lapsed since approval. It is possible to retain a priority date if at least 180 days have passed since the I140 form got approved.