Question: I'm just wondering if display:none is affecting my sites seo as I have for certain keywords dropped rank in google over the past few days since implementing this into a new homepage re-design.

I am using wordpress and displaying random post images on the homepage, getting the excerpt and hiding it using display:none and when a user hovers the image it displays the excerpt as a tooltip. 

I have read a few conflicting articles on this and watched Matt Cutts youtube video but that doesnt really give me an answer for the way I have implemented it. 

So I am hoping someone on here can give me a bit more insight as to whether it is not good seo practice or doesn't make much difference?

Thanks

Answer: 
Although you apply `display:none` style to your element, it doesn't matter with seo because your element dom still exist and what seo looks for is the dom and its content, not UI.
What i understand from your problem, is that google might black listing your website links. As hidden text/div are considered as Black Hat SEO technique and hurt your website very much.