I don't have the wk-1200 (mine is the wk 3000). But they are most likly similar. It is possible that you have 'turned off' the 'local' keyboard. This setting is ment for using the keyboard to control an 'external' sound device...and turning off the internal Casio sounds (headphone and speakers). You would want the 'local' option turned on. If that isn't it...try a factory reset (you should be able to find that in the manual....the manuals are also avalible online: http://world.casio.com/emi/download/en/manual/
Other posibilities could be having the keyboard 'split' set at an odd note (like all the way to the highest right side of the keyboard). The headphone jack could have gone 'open'...meaning the contacts aren't making, with the headphones unplugged (but that would'nt cut off the headphones)
Please post back what you find...to help others with similar troubles.
[This message has been edited by Jim Wintringham (edited 05-12-2005).]