Update!!!
Thanks for your answer, but reticently I studied how WP works and now I know that the mandatory plugin is installed. I cannot see it, but it is in /wp-content/mu-plugins as it is supposed to be.
And i remembered that ACF gave me the message to convert in PRO before I started to have problems, before the blank page. So this is not the main problem, I think!
So my question is: why I can see the page but not the fields? There is some necessary plugin to visualize them and I have to install it?