iMac Doesn't see Ethernet Cable I have a strange problem with my iMac (mid. 2011 21,5) -  I can't connect to the network using Ethernet because OS X tells me that it's disconnected when it's plugged in. Sometimes it can see the connection, but  the Internet won't work. 

Now, I also have this problem with Wi-Fi, it connects to a network but after random time Internet connection stops working. All other computers and phones are working with Ethernet and Wi-Fi; only the iMac isn't. 

I tried to do all tests and resets but nothing, today I formatted it returning from Yosemite to Snow Leopard but the problem is still there. Any solution?
Just for grins and giggles...

Did you verify that your cable and network port are good?  

First thing to try is moving your connection from whatever port on the switch it is on to another.  Sometimes a port on the switch just may be bad.

You will also want to verify the cable.  If you have access to another computer or a laptop, remove the patch cable from your iMac and plug it into the laptop.  If the problem persists, then you will need to solve that issue before doing more diagnostics on your iMac.  

Once you have verified that the connection from the switch to your iMac is good, then I would proceed with diagnosing the hardware, then the software on the iMac.