Did I destroy my 12 hour render trying to save it? I got up this morning and I went to save my overnight render, I turned on the screen and saw it for a moment, then I went over to Image to save it and at this point I think my mouse accidentally clicked on the "Open Cache Render" button and the whole image turned transparent/black, I didn't close the scene yet but I did move to another layer and did a quick 1 sample test render to confirm that doing this causes the same issue, the entire render is now just a flat transparent nothing.

Is there a way to get my render back?

PS: It's not the end of the world if it's gone, just wondering if there is a way to come back from loading up the "Cache Render", In the future I'm going to try and find a way to save the Render as soon as it's complete like for an Animation so I don't have to worry, although this is the first time I did this after countless renders so I guess it's just a freak accident.
If you didn't specify a place for it to be saved blender automatically places it in a folder on your C drive called tmp. C:\tmp\ you might find it there.