Thunderbolt not working

The thunderbolt bridge is inactive on my iMac (27" mid 2011) running Yosemite 10.10.4

If you connect anything to the thunderbolt ports it does not recognise them and remains inactive, i am trying to connect my thunderbolt display, the display does work when i connect it to my macbook pro, so not a problem with the thunderbolt display.

The thunderbolts did work when my iMac was on Mountain Lion OS and as only stopped when Yosemite was installed.

I have tried SMC & PRAM resets but has not helped.

Any ideas?