Keyboard funnies

My custom keymap seems to have gone funny.
It seems that it now only contains the keyboard shortcuts which I've explicitly defined (or maybe those added by plugins), and all the default ones are gone.
For example, the Up arrow now scrolls the screen, and CTRL-C, Z etc. don't work at all.

Any idea how this could happen?
And how to restore it?
The keymap XML file contains just my keys - not all the default ones.

Thanks,
Calum