The red X on your WiFi icon indicates that the connection is disabled. There can be a number of reasons for this.
Some things to check include but certainly are not limited to the following:
-Make sure the wireless radio is powered on. As someone else mentioned, there may be a keyboard shortcut that turns that on. Usually if it is a keyboard shortcut, there will be a blue or some other colored symbol on one of the function keys (F1 through F12) that looks like a radio tower with visible airwaves radiating off of the sides.
-Make sure the WiFi connection is not disabled. To do this, go to the start button, click on run... and type in "devmgmt.msc". (without quotes) and press Enter. In the device tree, look for Network Adaptors. Expand this section and look for your wireless network adaptor. If you can see one that is disabled, you can right-click on it and choose Enable.