Quite a delicate situation indeed! Losing a girlfriend that you still love is not something that men can stand and most of you will do whatever it takes to get her back again.
There are probably many ways but how can one know which the best would be in a certain circumstance? There are many situations and as many reasons why you can lose your girlfriend, according to the amount of fault each of you is responsible for.
Consequently there are also many ways to approach the situation.
Before taking action you must definitely think of the following points.

1. First and foremost think of what the breakup was caused by.
   If she is gone, you must have made her to; no one breaks a relationship out of the blue, for sure.
   Even if you don't realize what exactly brought about the terrible situation you are in, there must be a good reason for things to have developed like that.
   Figuring out what caused the rupture may be the first step into making up.
   
2. If it was completely your fault and you admit it in private, it is probably high time you started fixing the situation by openly admitting to have been a fool.
   Girls need that and if a man lets his woman know he is sorry for what happened and shows he understands his mistakes he stands a good chance to see the door opening for him again.
   Don't be afraid to say 'I'm sorry' if you really are.
   
3. While you are striving to get strength to court your ex girlfriend again and prove to her you have changed, you had better keep socializing with the people you feel good with.
   Friends may give you good advice or just be there for you to get over the critical period.
   If you happen to meet someone new in the meantime, you can go out with her.
   Being seen with another woman, friend or potentially more than just a friend may help your image.
   Jealousy can make people look back and turn around; your girlfriend might as well think it all over.
   
4. The best way to pass the time and get great results is trying to show your ex girlfriend two things at the same time.
   The first is that you are moving on (or at least that you are trying to) while the second that you still care about her.
   This paradoxical behavior is so manly and desirable that no woman will remain unmoved.
   
5. Finally, if you are currently not really friends with your ex, work hard to become friends again.
   This will certainly help building trust, and will get you closer to each other every day.
   Once you are sure of your friendship, you can ask her what she has on her mind and what made her run away.
   She might admit it was a mistake to leave you or just let you know it was a mutually beneficial move.
   As soon as you've got an answer, you can devise the proper plan to get her back or leave her alone.