iMac suddenly stopped seeing my WiFi network, which works with every other device in the house

Solution 1:

I had same problem with my Mac Mini. I changed network's name and Mac Mini recognized new network. It meant that issue was only with my Mac. I "forgot" this network

(Settings -> Network -> WiFi -> Advanced (OS X 10.10.3), selected missing network and removed it from Preferred Networks)

and connected to my network.

Solution 2:

I had the same problem, suddenly. Solution? I opened the list of saved networks and removed it, it appeared in the list of networks in range immediately.

Solution 3:

I changed the channel on my router to "auto-select" and it fixed the problem. Ran diagnostics on my iMac and it found "Crowded Wi-Fi Chanel" so this may be the case for you too. Try changing the channel on router or do auto select.

Solution 4:

Can you see it if you boot from the recovery partition? Try booting while holding down Command-R.

Once booted, try to connect to the Wifi.

If you can see it in Recovery mode, then it's probably a corrupt system file or cache. If you still can't see it in Recovery mode, I have no idea what the problem is.