1. Push the 'wake' key...
2. Push the power button.
3. Push and hold the power button for 4 seconds to shut down the computer, then power it on again.
You may have a sleep key on the keyboard, but unless there is a setting in the BIOS to wake up from keyboard, the wake key sometimes doesn't work.
To wake a computer using the keyboard, the standby 5v line (always on) has to be routed to the keyboard socket, rather than the normal 5v supply which is switched by sleep mode. Some motherboards require a jumper to be set for Keyboard wake-up and it may be that your motherboard doesn't have the jumper set correctly.