What does it mean when somebody says nobody is there for them and you reasure them and now whenever you see them its like they don't know you or like you or how do you ask what's going on

I think maybe u should just talk to them and again reassure them that you are there for them. Maybe they are going through something serious and need some serious reassurance that you are going to be there even after they try and push you away. Some people push people away just to see if they are real friends and will come back to them and not give up on them. So DON'T give up on them. 

Good Luck! :)