The hairstyle & makeup that is appropriate for a formal event is the one that flatters your face. An invitation to a formal event is very exciting and an opportunity to dress up. Should you wear a different hairstyle or makeup? You probably are wearing your hair in a style that you think looks good on you so why not stick with the tried and true. If you are wanting a new hairstyle and have time to experiment before the event you might try some new styles. Usually with medium or long hair you have more styling options than someone with short hair and you can try different ways to wear your hair before the formal event. However it is important to feel confident at every event. If a new hairstyle will detract from your confidence level don’t go there.
As far as makeup for a formal event you can wear what you would normally wear in the evening if it flatters you. If you want to ramp up your usual makeup you can have your makeup done by a professional. If you don’t ordinarily wear eye makeup you might want to have your eye makeup done for a formal event. Remember less is more when it comes to makeup. Too much makeup can make you look cheap and / or old. Below is a video by Michele Phan demonstrating formal event makeup.
The most important thing when going to any event is your attitude. Don’t try a new hairstyle or new makeup if you won’t feel comfortable.