He may:
1. die
2. commit a suicide
3. be killed
4. get in prison
5. have no money on his handy
6. loose your number
7. drink too much and lie on earth unconscious
8. have his mobile uncharged
9. take LSD and forget about you
10. format his mobile phone so that all numbers including your one are deleted
OR: he may just have no desire for calling you.