My cinema display stopped working with imac after a power outage. However cinema display still works when plugged into macbook pro.

My cinema display stopped working with imac (iMac (27-inch, Mid 2011)) after a power outage. However the cinema display still works when I plugged it into a macbook pro. I tried reseting the imac and removing the power cord for 15 seconds. The thunderbolt port on the imac also still works when I plug a thunderbolt hard drive into it. Ideas?