What to do if I was assaulted at work by another employee but my boss is doing nothing?

Your employer is not liable for the criminal actions of a co-worker, unless there was some reason the employer should have known that the co-worker posed a credible threat, and yet, despite that knowledge, negligently (carelessly) failed to take some appropriate action, such as terminating the attacker. So if this employee had attacked others at work, had made threats against you which the employer was aware of, etc., then the employer may be liable for their "negligent supervison"--but generally not otherwise.