I got back into the country after 2 months and my boyfriend does not call or answers my calls? What is up?

We have been dating for 8 months now, and things have been really good. I had to go abroad for some work for 2 months, but we kept in touch regularly. We would keep in touch regularly through phone, text and emails. Distance really got us more closer, i sent him postcards etc and he loved them. The day before my flight back, he even called me to say how excited he is to see me. When i came back I called him first, he didnt pick up I guess He was sleeping. The next day i ,messaged him and called he still didnt pick up. I even called his sister to find out if it was ok and if he had his phone, she said yea. I even told her to tell him to call me back. I got my phone activated and called him, messaged him still no answer!! It has been 7 days now that I have talked to him. I haven't called or text last 2 days thinking to give him some space and time. What I don't understand that how can a person change in one day? What should I make out of it and what is the solution, I really love him.