When it happens to me it is usually because the cable company is having network problems. Not real obvious because the cable modem looks OK, I'm connected to them but they're not connected to their next tier provider, or their nameserver is down.
But then the message is "Not connected to the Internet."
I seldom see "not connected to a network" because I always have an intranet running my own router downstream from the modem.
One tool I use for troubleshooting is my wifi tablet. If it is seeing the network and the Internet, then I can associate the problem with the PC. It sometimes fails to reconnect when waking up from a sleep mode, and I suspect the OS more than the hardware because a re-boot usually fixes it.