Can't add media to posts or pages

When I click on “Upload/Insert” to add photos, etc. to a page or post, the popup window is blank. I’ve tried on multiple browsers. The same thing happens when click on “Add New” from the media library — I just get a blank screen. I’ve tried updating & deactivating plugins in case something is interfering. I was able to add images yesterday, and to my knowledge, nothing has been added or updated in our backend since then. Any other ideas what the problem might be?