The wireless radio may have been accidentally turned off. Check to make sure that the switch on the front of the laptop is not switched off. On the left, there is a small picture of a monitor, and on the right, it shows a monitor with radio-wave like lines radiating from it.
Also make sure that you did not accidentally press Fn and F5 together at the same time, as this also disables the Wi-Fi. To find out, press Fn and F5 together and see what notification you receive. If the laptop tells you that you have disabled it, press Fn and F5 together again to undo this.