Try to go to the Phpmyadmin of your server, and change the selected theme…
you do NOT need to go into phpmyadmin and do anything. (sigh)
as long as you still have the default theme installed properly and available, deleting the other theme will not cause problems. WP is intuitive enough to display the default theme if another one is deleted.
Thanks, whooami. I’ve deleted my K2 folder and as you said WordPress then went to the default theme which meant I could log in. Brilliant. 😀