Reset, hold both home and power buttons for about 10 seconds until the iPhone begins to restart. If still black screen, possibly the LCD Display connector has come loose from the logic board connector and simply needs to be pressed back down into place. Or the front screen assembly is broken and needs to be replaced. If there is Warranty or AppleCare this will be covered, if no coverage, then with Apple this is handled with perfect Replacement iPhone 4 exactly like yours for $149.