Friend suddenly wants nothing to do with me...what is the best way to handle it?

I work with a former friend of mine, and it's really weird because he simply ended the friendship one day out of the blue. Things were just fine between us right up until the middle of our shift on Friday, and suddenly he just appeared sort of sad and not himself. I asked him if he was feeling okay, and he said he was fine. Today was even worse. He came in appearing quite sad and even though I asked him two or three times if he was feeling okay and why he was sad, he maintained that he was fine. Another co-worker and I were trying to joke around with him, but instead of laughing like he normally would, he looked extremely annoyed and I heard him curse under his breath. I asked him why he changed so suddenly, and he simply said he just wants to work. He said he wants to be my co-worker and nothing else. No friendship whatsoever. I went the rest of the shift without talking to him. So what would you do if you were me? How would you handle it when a friendship abruptly ends?