Adjective-- Completely engrossed in thought, absorbed, previously occupied, filled.
Verb--To absorb or engross to the exclusion of other things, to occupy beforehand or before others.
1)His lack of interest in this issue clearly shows that his mind is preoccupied with something very important.
2)"Peter, your preoccupation is not letting me concentrate", yelled the teacher.
3)"Hey Kiran, take this cold drink and release your mind from preoccupation and confused state", soothed Stevens.
4)Raju's mind is preoccupied with something else.