WiFi icon not showing on the menu bar and Internet not working (wifi or ethernet)? It used to work fine. There is no physical damage to the MacBook and the WiFi network I'd like to connect is is perfectly functional.

It will not connect to any WiFi network or ethernet-based networks. 

How can I fix this?

Sometimes, the wifi status is in the top bar, but when I click on it, it says "No Wi-fi Hardware Installed".
Is it possible you inadvertently disabled wi-fi and ethernet in system preferences > network?  If they are not listed in the left hand column of that window you can add them back by clicking the plus sign to 'create a new service' and then setup wi-fi and ethernet.  It's nothing more than picking them from the list of services and then clicking 'Apply'