ARe you talking about a basement ceiling, where the basement is BELOW ground, and the ceiling is separating from it down there? Because if this isn't the case, maybe you're using the 'wrong terms'.
OR, do you simply mean the ceiling is separating from the wall where both the wall AND the ceiling are ENTIRELY above ground? Also, which direction is the separation occurring? Top from wall where the wall is sinking? Or, Top from wall where the wall is 'falling away' from the house/ceiling? And how old is your home? And what do you have for a foundation (and I mean the foundation; not wall of the house ...)?