My wife has a Macbook Air (2013), Mavericks 10.9.2.
It connected wirelessly on Wednesday, but will not connect wirelessly or via USB Ethernet today (Thursday)
All our other devices (Android and Desktop Windows 8.1) have no problem, either wirelessly or hard-wired.
I have tried a shutdown/restart but still no connection (wireless and USB hard-wired).
System Preferences>Network shows no errors either wireless or USB Ethernet.
As far as I am a know nothing was installed on Wednesday and my wife would not be remotely interested in 'fiddling' with settings she knows nothing about.

All our hard wired devices are using powerline, but I have connected the Mac directly to the Huawei HG659 modem/router (VDSL) in the garage and still no internet access.

This is the first time this has happened since we got the Mac (Sept 2013).

Any suggestions on what to try next?