General:
* Absent
* Gone
* Lost
* Unaccounted for
* Vanished
* Disappeared
* Not present
* Not found
For people:
* Unreachable
* Out of contact
* Unseen
* Lost sight of
* Unsure of whereabouts
For objects:
* Lost
* Misplaced
* Out of place
* Not to be found
* Not in sight
For feelings:
* Yearning
* Longing
* Craving
* Lacking
* Desiring
* Needing
For opportunities:
* Missed
* Passed up
* Not seized
* Let slip
The best synonym will depend on the specific situation and the intended meaning.