wifi in MacOS Sierra

You need to update Wi-Fi preferences. Just remove the ones you have now. Click on any empty spot on the Desktop and press Command + Shift + G. Now, type in this directory: /Library/Preferences/SystemConfiguration/

In the folder that opened, delete these files:

com.apple.airport.preferences.plist
com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist
NetworkInterfaces.plist
preferences.plist

After these files are gone, reboot your Mac, together with your Wi-Fi router.

I have what seems to be the same problem, but the fix above does not seem to have worked. The only way I can reconnect to WiFi is to turn it off and on again on the iMac

Will this delete anything? Like known WiFi networks?

No, it didn't do any of that. All my WiFi connections remained.

This did not work for me. I do not have the file "com.apple.network.identification.plist." The closest named file is "com.apple.network.eapolclient.configuration.plist," and I did not delete that one. I'll research the issue some more.

I followed Method 1 from this post (http://osxdaily.com/2016/09/22/fix-wi-fi-problems-macos-sierra/)...essentially the same as Ella Fitzgerald's instruction above. And, this time I deleted the file "com.apple.network.eapolclient.configuration.plist" also. Seems to have worked.

Thank you, all.

Oops, I spoke too fast. It's still not working.