Blank site

After updating the WordPress to 4.2.2 version, I clicked ‘Appearance’ to update the contents. Then the Themes 1 page appears and says ‘ERROR: The theme directory “twentyfifteen” does not exist’.

Then I went back to out website and the front page has gone blank. How can I bring the website back? The website is https://www.yuzumanchester.co.uk.