My (93 year-old) Dad's iMac has suddenly stopped going out to the internet. I suspect that some System file has been corrupted.
It is a 2012 iMac, 2TB internal, 4GB RAM, running El Capitan (10.11.6) - I believe that's the last OS that will run on this hardware.
The network setting in the System Prefs shows that ethernet (wired) and WiFi are working fine. I tried FaceTime from my phone and it rang on the computer - which tells me that the basic connectivity appears OK.
However, apps that need the internet, just don't work. Browsers, SpeedTest, FaceTime calls out, Skype, WhatsApp - all don't work.
I tried switching users but got the same result - apps that need to go out to the internet just don't work.
I tried using my (USB stick with) El Capitan installer (to reinstall the OS) but I think the certificate has expired so the error message said to re-download the installer. I went to my computer to do that but the .dmg file gives an error ("Invalid checksum").
I went to OSXDaily (found via search) and followed the link to re-download the installer only to end up with an error (apparently from Apple) that the download is not available in Canada.
So I'm stuck and wondering if anyone has some ideas/suggestions on how I can get my Dad back online. At his age, he really looks forward to checking his email and WhatsApping family around the world - so there is some urgency to my request.
Before doing so, however, I'd install and run MalwareBytes for Mac and someone also suggested DetectX to ensure it's not a malware issue. You note that these apps "aren't connecting" to the internet, but details & screenshots might point out a specific cause.
I would start with the simple things: Power down the Mac, then the router. Power up the router, wait for it to be stable. If possible test it with another device (say a smartphone.) If that works, power up the iMac, log back in and see how it goes.
Thanks for your helpful suggestions and ideas, Folks.
Patrick - other computers on the same network are working fine. His is the only one not going out.
CubaMark - thanks (smacks face) ... I completely forgot about the Cmd-R option. I will try that before all else. I did run Malwarebytes (it's installed on the computer). I'll try and post some screenshots - I didn't do that earlier because they are not very helpful.
polywog - I did do that last evening (power down the iMac and router) but it made no difference. The router is an Apple Extreme.