No access to WordPress after changing theme

Evening,

I installed and set a new theme but now can’t do anything on my blog due to an error. The URL is https://www.nethermoss.co.uk/wp

I’ve tried renaming the theme in the hope that a default will be picked up, but with no success. I’ve also upgraded to v3.1 but that hasn’t worked either?

Any ideas?

Thanks for your help.