A few weekends ago, after a woman made a flamboyant scene over finding a hair (one that I believe was hers) in her dinner, another customer asked me, "What should we do if we find a hair?"
First, know that there are many sources of hair in food and that it doesn't do any good to point fingers at anyone. The hair could have already been in the food when the restaurant received it, it could have come from an employee or from your own body, or it could be a stray hair that happened to be on your clothing. If you at least allow the possibility that it might have come from you, say off of your sweater as the server passed the plate by you in setting it down, this might guide you in tempering your response.
Next, you should tell your server about the problem discretely. We want to know so that we can take corrective action, but the rest of the dining room would rather continue dining without hearing about it.
If it's not a big deal to you, you can remove the offending hair or eat around it. Or you can let us replace your meal. Or, if it is a really big deal and you cannot face the idea of eating further, let your server know that you prefer not to eat any more.
The absolute worst way to handle the situation is to make a scene in the dining room. Not only is it rude to the other guests in the dining room, but it will not endear you to the staff, whose role is to help you remedy the situation.
Here are two cases in point. Once a woman with long flaming orange hair chewed me out left, right, and center because she found a long flaming orange hair in her dinner. Why be ugly to the person who can help you? And then there was the piebald guy with the most crazy hair who made a big vociferous stink to me about one of his own very distinctive hairs on top of his dessert. He was so rude that he is not welcome back. These are prime examples of how not to handle the situation.
Then there was the lovely woman who discretely called me over and said, "The tip of a disposable pastry bag got loose in my beans. I work in a kitchen and I know how these things happen. I just wanted you to know." We would have gladly replaced her meal, but she was content just to put the little piece of plastic on her bread plate and keep eating. This is a good example of handling the situation where you find a foreign object, including a hair, in your food.
Just remember what the bumper sticker says: "Hair Happens." Well, it should say that. It doesn't happen very often, but it does. Remember that we didn't plan it that way and we want to make it right; causing a scene or yelling at us is only counterproductive.