People often wonder, "Where's God? Why isn't He intervening in this situation?" This question usually arises from a painful situation for which there is never an easy answer, and so I want to preface my remarks below with a disclaimer: I am not addressing every situation in which God seems silent in the face of pain. (A very helpful book to read is Philip Yancey's Disappointment With God.)
God's silence in Psalm 50:21 is based on His Self-restraint in response to human wickedness. Earlier He confronts the hypocrites among His people: "You hate my instruction and cast my words behind you. When you see a thief, you join with him; you throw in your lot with adulterers. You use your mouth for evil and harness your tongue to deceit. You speak continually against your brother and slander your own mother's son" (vv. 17-20). Unlike the case of Aaron's sons Nadab and Abihu, whom God struck dead for their sacrilege (cf. Ex. 10:1-4), or Ananias and Sapphira (cf. Acts 5:1-11), God rarely strikes people dead today. Why is this? Doesn't He know that people don't fear Him when He restrains Himself? That He loses their respect? And it's not like He expects us in the Church to be His executioners (a responsibility entrusts to the "state" according to Rom. 15:4).
No, like it or not, God's silence is based on His patience. "The Lord is not slow in keeping his promise, as some understand slowness. He is patient with you, not wanting anyone to perish, but everyone to come to repentance" (2 Pet. 3:9). His "silence" is the calm before the storm, the opportunity we have to repent. We must not waste it; it will not last forever.