Why won't my iMac find any wifi?

I have a June 2011 27" iMac. I7 3.4ghz. My iMac worked perfect in every way until Thursday May 14th. Then I came home from work, turned on my computer and there was a gray screen with red/pink stripes. Found out the graphics cards were faulty and apple had a recall on them. I just had the graphics card replaced on Monday (5-18-15). When i brought my computer home it would not pick up any wifi in or around my house. It usually picked up the wifi from my router, the neighbors wifi and my TV. It says its scanning but not picking up anything. I know my wifi is working because 2 ipads and 2 iphones are connected and using the wifi.

I brought my computer into the apple store Wednesday (5-20-15). they plugged it in, turned it on and it phone the stores wifi instantly and worked great. Apple ran their diagnostics test and everything checked out fine. I brought the computer home, turned it on, cannot find any wifi.

Here are the things I have tried:

Restarting the computer

PRAM Reset

unplugging everything from the iMac letting it sit 30 minutes, then restarting it

reset my router

changed router name

tried a different router

deleted "preferred networks to join"

created "new location" in the wifi assist screen

SMC reset

Different (not an apple) power cord

I am at a loss for words. The computer works with Ethernet, just wont pick up the wifi. Everything worked on it until I got the apple care repair.

Any suggestions would be greatly appreciated.

Thank you.