I made a big mess!
Everything started with the plugin Under Construction, which has been turned on for months because my blog (rclv.it) has been on maintenance. I was finally ready to put my site visible again, so I turned off the Under Construction plugin.. and nothing happened. I still got the "Work in progress" page I made a while ago. I read here that this plugin doesn't work very well with the upgrade to the latest version of WP, so I just deleted the plugin. Still only the "Work in progress" page was visible. Again, I read here that the problem might be caused by a conflict of the .htaccess file with the current theme. I deleted the theme and came back to Twenty Thirteen. Nothing changed, only "Work in progress" page was visible. So I went to my host and deleted WP, thinking that maybe it was just better if I reinstalled it again. I had to manually delete some more WP-files that were there, like the .htaccess and some php pages. I thought that then I could start fresh.
Nope. Even now, that I don't even have WP installed on my site, and of course the Under Construction plugin is long gone, I don't see a white blank page, I continue to see that "Work in progress" page.
Before I do any more damage I thought to come here and ask for help. How can I just erase what I did - especially that freaking page - and start over?