"no input file specified'

This is awful, and i am sure easily fixed.
But i am new to this scene…
I deleted the ‘wordpress’ word from the website in the ‘settings’ its used to read:
mydomain.com.au/wordpress
and i changed it to:
mydomain.com.au/
now when i try to access my account, this page appears:
“no input file specified”

How do i get into my account to fix this problem?

Many thanks,

Troubled WP User