Confused about a friend situation.?

Basically I have been good friends with my neighbours since I was young we grew up together. My friend used to come in a buy things from the shop that I worked and we had a laugh. One day last week my friend came in and I started talking to him and he went "I cant talk to you." I thought he was joking about and he stuck his hand up and waved goodbye and I thought it was just my friend being stupid. Then I see him again this evening and he says nothing walks out the door and puts his hand up again and knodded his head. I do not know what on earth has gone wrong because we were laughing about the good old days growing up and then all of a sudden my friend has stopped talking to me but is still civil if that makes sense.