That's kind of odd because I thought the same thing happened to me once because in the process of configuring my desktop, or updating the packages, it went back to double click. I set it once again to single click and haven't had any problems with it since.
In a terminal type, kwrite /home/your-user-name/.kde/share/config/kdeglobals
Ctrl+F opens the Find dialog in Kwrite, type in [kde] with the brackets. This is my [kde] section:
Alternatively, you can use Dolphin to browse to the hidden file -> kdeglobals, in /home/your-user-name/.kde/share/config. Open it in kwite and scroll down to the [kde] section.
You want to see SingleClick=true.
If you add that line and save, I'd backup the file first, and it still doesn't work, I might also try adding a new user and seeing if it still doesn't work in that account.