I found this in another thread
On the back of my iMac it is hard to see which way the Thunderbolt cable is actually suppose to be installed .. and IT IS POSSIBLE to connect it in the wrong orientation.
The FIX: verify your cable is connected to the iMac and Laptop in the correct orientation.
Also if you're sure the cable is correct you might try resetting the PRAM
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.