Try repeatedly hitting the H key and the Space Bar a number of times, each in quick succession.

I haven't gotten that since CS4, and at that time it was an older version of MS Entourage running in the background that was triggering the disabling the H key and the Spacebar to call up the hand.

If it doesn't work, you haven't damaged anything and lost only a few seconds. 

Same issue here. Spacebar just stopped toggling the hand tool in PS CS6 today. Not sure how to get it back.

Have you seen this thread?  If not perhaps some useful info.  http://forums.adobe.com/message/4411318

You may just have to reset the preferences if it work last week and you have changed nothing.  A re-install will not affect this file.

Start PS and immediately hold down the Ctrl + Alt + Shift keys.  You will get a reset window.  If you see splash screen you are too late.

I usuallly forget to mention trying rebooting computer as figure that would be first thing most people would try.  Rebooting, and turning off the computer when not using it, was SOP for the old guys.  But today many never turn them off, or even disconnect from internet.  Shows my age and resistance to change.

I don't think this is it, as I would bet that Adobe's software is smarter than this, but it's worth mentioning.  There's a bilingual keyboard layout feature built into windows that affects certain distributions of Windows (mostly ASUS and Acer but some other too) where they sort of pretend to have 2 keyboards even though they don't.  If it happens again, you probably accidentally tripped the keyboard switcher so just hit ctrl + shift and in theory, it would switch back.

There's no actual fix for it though unfortunately.