I notice you are registered in the UK, but you are currently signed in to the US ebay.com site.
That might be a factor in your seller hub display being unavailable or blank.
Try signing out here and signing in to ebay.co.uk and see if that allows you to see your listings. Try navigating to that page rather than relying on a bookmarked link -- eBay has made some changes to selling pages recently, so older links may not work as they did previously.
Try another browser to see if that makes a difference; if it does, try clearing your cache and possibly eBay cookies in your original browser.
If that does not help, you may want to post on the UK discussion boards in case there is a local angle:
https://community.ebay.co.uk/?nobounce=
