I installed kde 4.9 to see if I could go back to using kmail after a year o so (I uninstalled it because a mail client without autocomplete is useless unless you only write to one person).
The first time I launched kontact I was incredibly happy that it was working.
It lasted one reboot.
Any ideas?

Can you give more details? Did you change configuration between the reboots? Are there any errors in .xsession-errors? Did you switch on or off Nepomuk?

My bad.
Apparently it had nothing to do with kontact o kadressbook. . The whole plasma thing was crashing and fainting.
I waited for a few days to close the post to see if it happened again, but it didn't, so it was evidently a misdiagnosis.
Sorry.