My original problem was that I couldn't create a backup of my blog. I got the error message "backup directory not writable". So I came to the support forum, searched for a solution to that problem and found a thread where experts recommended changing the CHMOD settings of the wp-content folder.
So I tried to apply this piece of advice. Using WS_FTP95, I accessed my remote WordPress folder, right-clicked on the "wp-content" folder and chose "CHMOD" in the context menu. There I clicked the "write" option for "Group" and "Other", then clicked "OK".
When I returned to the browser tab where I had the WP-admin panel open, it said "page no longer valid", so I logged into my wp-admin again ... but there was no "backup" button any more - and - far worse - whenever I click on "view site" now or enter the blog URI in the address field of my browser, I get a blank, white page! My blog seems to have gone.
Luckily, when I click on "manage - posts" the posts still seem to be there. But nothing is shown at the blog address. Neither can I access the wp-content folder via FTP any more (error message: "Permission Denied").
What can I do?
I'd be most grateful for helpful advice.
And another question: if backing up is such trouble - is it even worth it?