Why do guys just stop talking to girls they are involved with?

I was hanging out and sleeping with this guy for a little over a month. Things were going good, I was very enamored with him...then out of nowhere, with no explanation, he just stops talking to me. Just stops. It's been like 2 weeks since we've spoken. He doesn't answer when I call, and does not respond to my texts. Why would a guy do such an indecent thing?