If it's someone you love dearly then you can always try and apologize if it was your mistake. If the person has done some wrong to you, then see if they are repentant about it and asking for forgiveness. Forgive them, if possible.
However, if the friend is not interested in you anymore and is avoiding you, then there is not much you can do. There is no need to feel sad about losing such a 'friend'. True friends forgive and forget and will be there in your time of need.
Nothing in life is worth crying about.
Cheers.