There are only a few options when this happens, in my experience:
1. If your love is deep and real, you may try being the person's friend and hoping that love develops over time. The advantage to this is that you get to spend time with the one you love. The disadvantages are that while you are focused on this person, you may not notice other opportunities for love, plus it can be very painful to spend time with someone you love and have them treat you as a friend. This option is for if you are convinced that this person is your soul mate.
2. Your "love" may be infatuation. If you have made honest attempts to engage the other person's interest, consider moving on. This may have the effect of causing the other person to realize what they are missing, or at least it will set you free (after some sleepless nights) to seek new love.
This is not an enjoyable situation, but it is often part of being human. Remember that we all deserve to love and be loved. If unrequited love keeps happening to you, you may need to look at yourself and determine why. But that is a topic for another day. ; )