Karl writes, "My mother, a United States citizen, has filed a permanent residence sponsorship application for me. Due to mailing address problems some important notifications were lost and my application has been delayed. In the meantime, my work permit expired. Now my employer has refused to pay me for the work I did and is saying that since my work permit expired a few months ago I had been working with them illegally for three months. He even threatened to report me to ICE that I was working illegally. Have I committed any crimes? Please I need your advice."
Working without authorization is a serious crime and it not only puts you in trouble but also the employer because an employer must make sure that they only hire and employ legal workers at all times. As an immigrant with a time-limited employment authorization, it was your duty to renew your work permit on time and let the employer know when it expired. Since you did not do so, the employer can clearly accuse of hiding facts from them and misleading them, things that employees are expected to do and depending on the papers you signed when you joined the company it was probably even a requirement between you and your employer.
You could, of course, still sue the employer and get into a long and complex legal case to get back wages but considering that you are not truly "legal" it will just mess up things for you. So the best thing will be NOT to work any more until you either get your green card or at least have a valid work permit. Then just wait for the permits to arrive and start a new life the right way in America. In the meantime leave your employer alone and do not bother them. That way, it will just be that both you and your employer would let a genuine mistake disappear.