Exporting to PNG from illustrator - blank image

Hi

I've updated to the latest version of Illustrator and now when I try to export my file to PNG there is nothing there. 

So I go 

File> Export 

Change type to PNG

Select the artboard I want

Click Export and then in the freeview there is nothing there. I've tried with white background, transparent. I've tried closing Ai and reopening?

Help please