I am a 24-year old woman and have been single my whole life. I am average looking and am not particularly overweight or ugly. I have never been approached by any guy though, either in school or college. I don't know if this is because I come across as unapproachable, because I have always been easy-going and don't think I am difficult to chat with. This didn't bother me much at first but is beginning to play on my mind because all my friends and classmates have been in relationships and I don't know if I will have one. Is there anything I can do to change this?
There is nothing abnormal about your situation, because meeting someone has more to do with just being approachable. You may simply not have exposed yourself to situations that encourage communication with guys. If you think you aren't being approached because they misunderstand who you are or what you are like, you can always reach out to them and initiate a conversation. You can ask friends to introduce you to new people, use an app to meet and go out with people, or simply have more conversations that allow you to get to know more guys. There is nothing wrong with you, so the best thing to do is have patience.

I have a number of colleagues at work who hit on me despite the fact that they know I am in a relationship. How do I get them to stop?
A lot of men seem to have a mistaken notion that hitting on someone will change their mind about who they are dating. If this is a problem, you have the option of ignoring them, asking them clearly to back off, or getting in touch with your HR department. There is no excuse for any sort of behaviour that makes you feel uncomfortable in any way, and 'hitting on' is not okay if it prevents you from feeling comfortable.