Problem daisychaining displays with Macbook

I have been daisy chaining x2 thunderbolt displays with my retina MBP for around 12 months with no issues.

About a month ago, one display blacked out. They both work independently without a hitch, but simply will no longer daisy chain. Essentially the screen that is directly connected to the MBP works 100% of the time, but the additional screen connected via the initial screen does not. What is strange is that the sound will only work on the screen that is black.

Very frustrating when I need to use them both for work. What is strange is that both work fine with my Macbook Air.

t doesnt show up as having two displays in my preferences even though the sound still works from the blacked screen.

Im not sure if this has something to do with upgrading to Yosemite, Im pretty sure the issues started around when I did upgrade.

Can anyone shed any light on what I can do ? I

Thanks in advance

Dan