Engine doesn't start, only clicks...

[bunskippy]

I have a (recently acquired) '68 Coupe Mustang w 2v 289. Hadn't started it in a week or so, and now it just clicks when I turn the key. Strangely, the aftermarket radio (which is not tied to the ignition) turns itself on. Electricals seem to work ok (lights, etc.) but it just clicks, nothing more.
It used to start just fine, after a few seconds cranking, pumping the gas a bit, etc.

what could have happened -- what can I do?

thanks.