I can't stay connected to internet. I've changed my location. Same thing. All my other devices are working fine. How can I get my iMac back online?

I am unable to connect to the internet. WiFi, ethernet all working fine. Multiple devices can connect to the internet. Diagnostic on iMac says I'm connected and should be able to access the internet. This is all beyond me. I've changed the location, it appeared to set every thing up. Still doen't work. It must be corrupted so how do I clean it up?