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Please tell me what OS version is on your MacBook.
Make sure the wireless switch is On, if your MacBook has an external switch for enabling/disabling the WiFi. I presume that isn't the problem though, since you have run network diagnostics, which would have advised you if your WiFi was disabled, I think.
Otherwise, depending upon what OS and OS version you have, there could be other problems... so let me know that detail and I will continue to advise you. Thanks.
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