When Celie suggests that God "must be sleep", she is referencing all the bad things that have happened to her and implying that if God's role is to watch over people and guide them through life, then God must have fallen asleep to let all these things happen to Celie.
Being raped by the man she grew up believing was her father then having her children and only friend (her sister Nettie) taken away from her, Celie feels that very little good has come from her life. If God was the kind of deity she had been raised to believe in, these things would not have happened. This is what Celie implies in statement.
In the short chapter, the last the Celie addresses to God, Celie reviews these circumstances and a few others with straight-forward brevity and ends with the line:
"You must be sleep."
Thus Celie ends her correspondence with God. Soon after this point she invests her spiritual sensibility in a new conception of God, one close to Shug's idea of God as a faceless force in the world.