If you plugged everything but the machine doesn't boot, please take a look inside Raspberry Pi case. There should be two LEDs: a red one and a green one. In normal conditions, the red LED should be on and the green one should be flashing; if the green LED is not flashing, it means that there is problem during the boot.

Try to unplug the power supply and wait at least 30 seconds, then plug it again. The green LED should start blinking, even if the screen remains black. Give it a couple of minutes, if the system wasn't properly shut down, the Raspberry will check the disk and try to repair any corrupted file.