Why would a person that I met on the internet stop calling? Details inside.

I met a guy on the internet a month ago. We hit it off and he started calling me everyday and we talked for about 5 hours a day or more. We discussed meeting and if that went well, then taking a vacation this summer together. Then, without warning, he stopped calling and I haven’t heard from him since. I called him once and left a message via email. He never returned my call. I know he is still alive, I have seen him on a website. So what would make him stop calling? We never argued about anything and really seemed to genuinely care about the other’s life and feelings.