Unable to modify slider images

Hi, My website: http://www.dragonvet.co.uk has had two slider images for months now. Yesterday I tried to add two more sliders via the customiser. As I added the second image and some text the customiser seemed to hang for ages. I logged out of WP and logged back in. Now when I re-enter customiser it doesn't load properly (the website title says "Customise: Loading" constantly) I can now only enable or disable the slider, but it's not showing me any options to modify them or add any more text. Thanks, Ben