1. Keep your appointment with the investigating committee. Your manager, HR, and any other neutral party could be involved. No other meeting can be more important than this one (barring personal exigencies). Your job maybe on the line. Avoiding this meeting will put you in a bad spot, and like it or not, be viewed as insubordination. If you must cancel, which you should do only if you have a really unavoidable reason, reach out to the other parties and explain clearly. Try doing it in person and propose an alternate time.
2. Listen. Whether this is the first time you are hearing the accusations, or you have already been given a heads-up before going to HR, be cautious and listen to what you are being accused of. Often people are so ready to object and prove they are innocent, they fail to comprehend what is being discussed. So pay close attention to what is being said and what isn’t.
3. Consult a lawyer. Set up some time with an employment lawyer, to understand what your next steps should be. Professional help, although expensive, could be useful in salvaging your reputation and/or deciding on your next steps.
4. Share your side of the story and offer proofs. Depending on the nature of investigation, HR may or may not share the name of the complainant(s). But, you would want to understand the context of the complaint to be able to defend yourself. So ask for as many details as possible. If you are able to gather eye witnesses or show documented email trails, make your case. Be completely truthful and cooperate during the investigation.
5. Do not retaliate. If you know who has complained against you, do not under any circumstance try to get back at them or retaliate in any manner. That will just make your case worse, and can be grounds for disciplinary action up to and including termination.
6. Ask to understand your options. If the investigation proves your innocence, then you go back to your normal job, if not, know what your options are. Can some training help? Could you resign, instead of being terminated? (Note that if you choose this course, you might not be eligible for unemployment benefits.)
If you are guilty, know that it will go in your file, even if you are given the option to resign, and you maybe blacklisted from approaching the organization for future opportunities. What you can do, however, is request that your employer to note your exit as a voluntary resignation so your future employment with other organizations is not hampered. Know that they may not honor your request. If you have been wrongfully terminated, i.e. if the investigation was not thorough and you were not given a chance to explain yourself, you do have the option of choosing legal recourse.