An employee has accused her supervisor of sexual harassment, and just informed you about it.
Now what do you do?
First, call your employment attorney, who will likely recommend an independent workplace investigator to interview all the relevant witnesses and work with that attorney to develop recommendations as to what, if any, steps should be taken.
Conducting your own investigation is fraught with peril. First, you don't have the experience or knowledge to do so, and secondly - you can avoid liability by having that investigation conducted by a neutral third party.
Of course, the first order of business is to make sure you have non-harassment and non-retaliation policies in writing immediately.