Admin page went blank

Try:
– switching to the Twenty Eleven theme by renaming your current theme’s folder inside wp-content/themes and adding “-old” to the end of the folder name using FTP or whatever file management application your host provides.

– resetting the plugins folder by FTP or phpMyAdmin.

Thanks.
I love to try it but I don’t exactly understand. I’m already in the FTP server and I in themes (where I by the way have several uploaded theme, on top of the used one). The one I use is Colorway. Should I rename this folder?

Yes.

And I should call it Twenty Eleven? What was this “old” you where mentioning? I don’t risk to loose any content when I do this?

No – do not call it twentyeleven. Call it something like colorway-old.

Ok I tried it and it worked! I have my admin display back. So now I just swith to any other theme? And if I want to use the same theme as before, I reinstall it?
Thanks!

So now I just swith to any other theme?

Yes – any apart from your previous theme.

And if I want to use the same as before, I reinstall that them?

Yes. Delete the theme’s folder using FTP and re-upload a fresh copy.

A last question, I’d like to work on the page without it being public yet. How do I do that?

Just one page? Or the whole site using a new theme?

Actually the whole page. I manage it on my own by using the Maintanance Mode plug-in.

Thanks for your support, very helpful!