Recently I ran into a nasty situation on Kubuntu desktop where a host of crucial KDE components would cease to work. Chiefly amongst them were:

1. No instances of Dolphin would start
2. The kickoff launcher would not work 
3. The KDE activity manager was crashing. 

I suspected the Nepomuk might be the culprit and I was right.  I stopped it by running

qdbus org.kde.NepomukServer /nepomukserver quit

I then wiped out its data by running

rm -r .kde/share/apps/nepomuk/

Finally I got it started again by runing

nepomukserver

…and my problems were solved.

Trying to search for a solution to this problem, I found people suggesting that one would wipe the ~/.kde folder and start afresh. That sounded too drastic and a great deal of my customizations would have been lost in the process.