Can't add pdf?

I am unable to add pdfs to my website. I used to be able to do this but all of a sudden this function has stopped. When I highlight my content and click on Upload/Insert, a blank pop-up shows up.

I have read enough about this issue to try deactivating all of my Plugins. This did not help at all so I am stuck. I have not been able to find any other posts about this so need some help.

I am using the Twenty Eleven theme and am just the end-user. I did not create the website but only maintain it and have no developer or code background.

Thank you for your time!
Kate