Site and dashboard are now just a white screen after changing themes

I’m beginning to feel like you problem child today. I went in to change the theme for one of my subsites : and next thing the back end and the front of the site are just a white page. I can’t change anything. I tried to edit the site and deactivate the theme and apply another – and no luck. I also deactivated all of the plugins. Is there something I’ve missed?