If you have made the complaint in the investigation, the investigator is going to want your full version of events. They are also assessing your credibility, so there is no reason to tell them anything but the truth. They will ask you questions, so make sure that you answer them honestly and completely to the best of your abilities. Their job is to gain an understanding of what happened, and you are one of the best people to help provide that information. They will also be speaking with any witnesses to get their version of events, and see how they align with yours.
If you have been accused or are otherwise the subject of an investigation, you have a right to know the allegations that have been made against you. The meeting with investigators may not be welcome, but it should not come as an out-of-the-blue surprise. You should know going into that meeting what you have been accused of so that you have a chance to jog your memory, and review a calendar if needed.
The investigation is a chance to get your side of the story, so you too should answer the questions fully and completely. If you recall events differently, or have other evidence that you can provide, that should be given to the investigator. Their job is to assess everyone’s credibility, and yours especially will be under review, so staying calm and cooperative through the process can often be helpful. Your participation though is key. Otherwise you can think of an investigation like someone suing you and you failing to defend yourself – it’s more than likely that a court would find in their favour.