There have been a few friends in my life who I've heard guys label "undateable." There are a different reasons why a girl could be labeled this way. Fortunately, if you fall under one of these categories, you can make changes to be perceived differently.
The Party Girl: the most common reason a guy will say a girl is undateable is because she is constantly partying, getting wasted and making an ass of herself. We all did it in our early 20s but that was 10 years ago, for me. Have the occasional wild night but make that once in a while, not every weekend. A lot of this is about knowing your limits. If you know that drinking a whole bottle of wine to yourself will cause you to fall off a stool at the bar, then tell yourself you're only allowed 2 glasses. If taking that shot of tequila will put you over the edge, don't take it! You might be a really sweet girl who is looking for a great guy but that great guy might not want to date a sloppy drunk. It's all about how you are perceived. If you're out at an event where there are eligible men, always limit yourself to 2 drinks. You want to be able to hold a good conversation and appear dateable. Think about who you are looking to date and ask yourself if you emulate those same qualities. If you want someone who is serious about settling down and wants to get married, would you go for a guy who was clubbing every weekend and getting wasted all the time? You'd probably perceive him as a party guy and not marriage material.
Another reason I've heard guys label a girl undateable is because she is too aggressive or negative. She's the girl who gets angry early on because the guy said or did one thing she didn't like. Or maybe she sends angry drunk texts. A lot of this is letting past experiences get in the way with someone new. It's expecting the guy to disappoint you before really getting to know him. It's assuming he will be the same jerk you dated last. Remember that you're the common denominator and look back and see if your patterns could be the reason the last 5 guys you met ended things early on.
All of these things can change. If you're truly looking to find someone, spend your time wisely. Figure out who you are looking for and list where you could go to find him out in the wild. Join Meetup groups, volunteer, go to networking events and drink two drinks or less, smile and give off positive energy. Be the person you are looking to meet.