Cannot turn wifi on on imac

If the WiFi status stays red, that means either that it is not connected, or that it is turned off. Or possibly that it is broken.

You might want to run hardware diagnostics on your Mac. I don't know whether or how thoroughly these test the WiFi, but what the heck. Also run the WiFi diagnostics, though I don't know if they will gather much, if any, information, if the WiFi won't turn on, or won't connect.

Use Apple Diagnostics to test your Mac - Apple Support

Please do the following in this order:

1. Restart Modem
2. Restart Router
3. Restart iMac in Safe Mode and then restart normally and attempt to join your network. Ensure your Wi-Fi is the #1 router in the list.

Here's something else you can try, if you can connect to your WiFi router, but you're having trouble getting out to the Internet. This diagnostics tool doesn't fix problems, but gathers a lot of detailed information which somebody who is familiar with WiFi and networking might be able to use to narrow down a problem.

Use Wireless Diagnostics on your Mac - Apple Support

If you go into Network Preferences and click on WiFi in the list, what does it say about WiFi?

Does it indicate that WiFi is Connected (with a green status "light")? If so, the problem may be with your Internet configuration (or lack thereof), not with WiFi per se. If you go into the "Advanced…" configuration and click on the tabs, you should be able to see if your Mac has an IPv4 Address, a Subnet Mask, a Router address, and numeric IP addresses for one or two DNS Servers.

Normally you'd get those things by setting "Configure IPv4" to "Using DHCP," so your Mac would ask your router to help it get proper Internet settings.

Another thing to check in "Advanced…" WiFi preferences, under the WiFi tab:

Do you recognize the names of the networks in the "Preferred Networks" list? These are networks that your Mac will try to join automatically. If you have accidentally joined a neighbor's WiFi network, the neighbor's router may not be set up to provide IP configuration parameters to your Mac … and it may even be set up to block access by your Mac (or any other machine with an unknown address).

If you see a network that you don't recognize, you may want to click on its name, then on the "-" to make the Mac forget about it, and not automatically try to join it again.

Please restart in Safe Mode and then restart normally and test. Also, it would be wise to restart you modem and router too.