Observe this person around other people. Discern if they act the same way towards others, because sometimes you can overreact or misread people. Maybe this person ignores everyone and just does not like social interaction.

Examine your behavior towards this individual. You may have done something inadvertently to upset this person. Think about when the silent treatment started and what happened in your relationship around that time. It could be a simple misunderstanding.

Talk to common friends and ask them if they noticed anything different about this person. These friends can give a different perspective on the situation and help you understand if there is anything else going on with the person that you did know about. For instance, they may be having trouble at work or school that caused them to withdraw and it may have nothing to do with you.

Talk to the person that is ignoring you. Ask them in a confrontational manner why they are ignoring you. Further, ask them if there is anything going on their lives that they want to talk about with you.

Listen to what the person has to say and take it to heart. Too many times people take offense at another person's actions and block out any criticism because they are offended or hurt. No one is perfect and everyone makes mistakes, but the tragedy is not learning from and correcting those mistakes.

Apologize for anything that you may have done to hurt this person. More than anything else, a heartfelt apology may remedy the situation and get the person to stop ignoring you.

Stay out of this person's way for a while. If after talking to them, they have no idea what you are talking about, and then just avoid their presence. They may just need some space, but even if they don't stop ignoring you then you are better off not knowing this person.