Why didnt he call me back?

so i have been meeting this guy..and he seemed really interested in me..he wanted to know everything about me..i could tell he liked me..used to call me at night.buh now he has been really busy with work as he is doing a really hard course and so he doesnt get time to talk much..however whenever he is online on msn most of the time he always makes it a point to message me..yesterday i called him as he wasnt feeling well but he didnt return my call..i am kinda paranoid as i dunno if he lost interest in me or just been busy with work..please help!!