Why is she ignoring me!?? Plz help girls!?

I've known this girl for 3yrs , we rarely meet each other n she loves in my building , bt whenever we meet, I ALWAYS catch her STARING and MAKING EYE CONTACT , literally always!!. She seem to be interested though, bt now shes ignoring me, she didn't reply to my fb msg neither she tried to talk. About a month ago, I saw her in a school fete, she was wid her frnds . on the other hand , me n my frnds were following her, she gave an angry look n walked away , I was jst trying to say hi bt she caught me following her, since then she didn't relay to my msg. What should I do now?? Y is she doing so!?? Plz help guys!