Solution for all times: Just ask for a new one.
A new strand of hair? I'm not sure how that's going to help.
Seriously though. It's not always just about the cleanliness of the hair (which is honestly relatively harmless in terms of the range of things which could fall off a human body and into your food if you stop and think about it). It's about the perception of the food preparation itself. If I'm in some cheap place (or fast food), I'll take it to the manager and just ask for my money back. At low level establishments, it's a good rule of thumb to *never* complain about the food and then eat anything they provide you with as a replacement. The odds of anyone in the kitchen taking their profession seriously enough to not put something far worse than a hair in your food at that point is too low to take the risk.
At any sort of "real" restaurant, I'll call the server over (cause they actually have them) and point out the problem. Their response will somewhat determine my response. If the immediate reaction is to offer me a complete replacement of my meal, I'll accept and be perfectly happy. If the reaction is to downplay it, or in any way do anything less than that, then I go into super irate mode. Trust me. You don't want to see this if you are a server.. I tend to be pretty laid back about a lot of things. I don't tend to complain about food, even if I don't particularly like it, unless there's clearly some sort of mistake that has been made. So while there are some restaurants I'll tend to avoid returning to because I didn't think the food was worth the cost, I'm not going to make any extra demands or complain about it while I'm there. I figure it was my choice to eat there, I've learned that was a bad choice (for me, but maybe not for others), and move on. I'm pretty darn polite in this regard. Right up until I decide it's no longer time to be polite. And it takes a server doing something I consider completely unacceptable, and not replacing food in which there is clearly something wrong is at the top of a pretty short list for me.