Answered by Susan, Hiring Expert at Praxair, Inc., on Monday, October 15, 2012
What an uncomfortable position to be in! Unfortunately, these kinds of things do happen, and if nothing else it allows you the challenge to test your ability to deal with uncomfortable situations and to act professionally, regardless of your Superior's behavior.
Share the email with your Senior Human Resources representative, and ask what process will be used to investigate. If you do not receive any update you should ask again, or escalate to the HR Director or higher.
I don't know what kind of company you are working for, but I will assume they are responsible and fair, and therefore the situation should be dealt with appropriately. However, assuming you and your Supervisor stay in the company, you should expect a difficult relationship for a period of time so best to ask the HR department for help in how to manage this professionally.