You are absolutely shattered! Your ex dumped you for no apparent reason and simply walked out.
You have mixed feelings about this, you are heartbroken, but at the same time you are also furious with him/her for treating you this way! You don't know whether to try getting your ex back, or to get back at your ex for putting you through all this misery.
Obviously you feel this way - who wouldn't in your position.
The only thing is opting to do either of the things you would like to do at this point in time will only bring you more misery.
If you try getting your ex back now, it will simply not work, because your ex doesn't want to talk to you right now.
And, if you try getting back at your ex, it will definitely backfire on you as well - you will look like a weak, pathetic, person who cannot accept the fact that your relationship has ended.
Put any thoughts of making contact with your ex right out of your mind - nothing positive at all will come from this.
It is your ex's right to break up with you for whatever reason, so just accept this.
You need to show your ex that you do have good qualities, and one of them should be to be mature about your situation.
If you're with your friends, DON'T say bad things about your ex - they won't appreciate it and it will get right back to your ex and put you right back where you started - being without your ex.
However, if you can say good things about your ex, then this will be a point in your favor - with your friends as well as your ex.
You need to make your ex regret dumping you, so over the next few weeks, do whatever you can to raise your ex's opinion of you and see you in a favorable light - this will raise your chances of success in getting your ex back.