iMac will not find network

We just got our iMac repaired (problem with the LCD screen), and when we brought it home, it would not connect to our network or even find the network. We have since taken it back to Mac Authority twice, and both times it connected just fine to their network. We even bought a new router, thinking that was the problem, and it still will not find any networks. I am on our MacBook Pro which is connected to the network, and I'm right beside the iMac, the same distance away from the router. We never had connection problems before our screen was replaced, and since the iMac connected to Mac Authority's wireless network, I'm at a loss as to what the problem would be. My husband and I have tried everything and done countless internet searches to try to figure it out, without any success. Does anyone know what the problem might be?

Is Mac Authority an authorized apple repair place? It sounds like when they replaced the LCD they did something to the Airport (WiFi) antennas. The reason it works there might be their WiFi signal is very strong. This is just a guess BTW, but can't think of any other reason.

How far is your router from your iMac?

I agree with dtravis7's assessment. Since the antennas are located in the display panel, it may be that they were moved from their original positioning when your LCD was changed out. Correct antenna orientation is important in both the iMac and Mac notebook computers. Misalignment can cause the signal to drop off considerably.

Yes, Mac Authority is authorized to repair Apple products. The iMac is directly a floor above the router, and the thing is, we never had connection problems with it before we got the LCD repaired. So I'm guessing we are going to have to take it back yet a third time and have them take the screen off and look at things. Nothing we've done has helped us get a connection.