Thanks for the answer!! See my response in your message.
Alex.
I won't likely be much help, but more information will help.
Oh, but first... Standard response: check the file permissions. Especially since you were in FTP, you may have accidentally changed the permissions, etc.
Make sure nobody else was in there either. You mentioned you have other people updating it. Be suspicious

Check your server logs. And clear your cache.
Is this my server cache?? How do I clear it?
Check your host's news, they may have changed something. Look for your php.ini or httaccess files to see if they have changed.
Now, are you saying that
1) all the WB administration pages are blank or
2) the front-end displays blank pages to visitors?
I am saying #2!! There is no more problem with the WB admin pages, I see those fine. But the front-end displays either blank pages or 403 Forbidden error page. The 403 error I can fix easily by change the file permissions. Now for the front-end displaying blank pages, see below my answer for more details.
If it's #1), I would try reinstalling again and see if you see anything before you restore the database. (actually, I'd probably do that, regardless)
It's not #1.
If it's #2), can you restore the database and then:
Yes it's #2.
a) see the Pages and content in the Pages administration area, it's just not displayed to visitors?
Yes that's it!!
b) check the database and see if your content is there
Yes it is there!! I did a db backup, so after reinstalling WB from scratch using the install script, I delete the emptey db WB had created and restored my old db with all the data in it. Everything is there still.
c) if a or b is true, but the output is still a blank web page for visitors, view the source in the browser and see if there is anything there.
The source is blank as well!!
- if your text content is there but invisible, the colours or division placement could be messed up.
- if your content is not there at all, but some HTML is there,
- check the index.php file in your template directory. There should be an include content(...); statement in the index.php file.
- if there is no HTML at all, and your index.php file is there... I have run out of ideas at this point.
My situation seems to be the last one you mentioned. The front-end displays blank pages and the source of these blank pages are also blank!!
I truly hope you can get some other ideas beyond this point!!

I will try to think of some myself and will update if I get something new.
Thanks!
Alex.