Since yesterday, I'm facing an empty "Desktop" on startup.
All widgets , icons etc ..or a different view like "Folder View" seemingly disappears after a reboot.
What could be causing this ?

I don't know if the errors indicated here are related, so here is my .xsession-errors.
http://pastebin.com/tuNaWDxM

Also, is there an option to reset all KDE related config files to default, so that I can be sure that this isn't something I messed up while tinkering ?

I saw a bunch of Nepomuk related errors in the log, but Activities should work independently of that. Do the old activities show up if you bring up the activity controller (Meta/super+Q)?

No, only a single one called "Desktop" is there.
I've deleted the other disabled ones ("Photos", "Search and Launch" etc) that come with the default.

(at least from my experience, unless they've changed this in versions more recent than 4.5) plasma only saves its settings on successful exit, so if plasma crashed (on exit, perhaps) for some reason then it won't have saved anything. For this reason (and plasma frequently crashing on my machine - I'm looking forward to more stability when we get around to upgrading to Ubuntu 12.04 w/KDE 4. I've gotten into the habit of running: "kquitapp plasma-desktop; plasma-desktop" to save any changes I've made.