Sleep key on my keyboard

Hello, I need a little advice. I have a new PC with Windows 7 and I bought a Logitech LX710 wireless keyboard/mouse... on the keyboard, there is a sleep key that I accidentally pressed (I know it's silly, but I wanted to try it!). The problem is that the screen went to sleep and won’t wake up! I have tried turning the PC on manually twice, and the screen goes to sleep immediately. I'm lost because I'm not very skilled with computers! Please, I need a little help. Thanks in advance.

Configuration: Windows 7 / Internet Explorer 9.0