What happens if my employer withdrawing money from my check to cover 401k contributions but then not making the contributions?

It may well be breach of an employment contract. It is certainly grounds for civil liability--your employer may not simply take and keep your money, but you instead may sue to get it back (plus interest and certain other costs or losses). It may also be theft--i.e. criminal activity. Someone at your employer may be stealing money from employees and diverting it to his/her own use or the company's benefit. If you your employer does not clear this up immediately and also give you a good explanation for what has happened and why, you may wish to both retain a lawyer to sue them and bring this to the attention of the police. (You might also check with other employees--are they, too, having money withheld but not contributed.) Act quickly: if there is theft, or if the company is failing and the money is being  used to prop it up, the longer you wait, the greater the chance there will be nothing left to recover.