fatal error after theme installation, cannot open dashboard

hello, I just installed a new theme named “sliding-door.2.6.1.zip”. When I activate it, this error comes:
Fatal error: Class ‘Walker_Nav_Menu’ not found in /home/content/45/4865745/html/blogs/wp-content/themes/sliding-door/functions.php on line 123

Now I can visit the site but I can’t use the wp-login part!!I can’t see my dashboard screen so I can’t change the theme!!
I deleted the theme from my ftp but nothing changed..

I have WMPU so I can login by using my other blogs. I can use “edit” part of this blog. I wrote different theme name to the themes part of blog options part. But nothing changed.. please help me!