Have a look in Device Manager first to see if there are any problems with the wireless device or it's driver. If there is a problem, there will be an error icon showing on it (eg a question-mark or exclamation-mark).
It may be you just need to re-install the driver for the wi-fi network adapter. You can download that from the Lenovo Support website: http://support.lenovo.com/en_GB/
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