When and how did this start happening? I know that some speaker systems (my logitech ls21's for example) have one speaker cut out after you use the headphones and then unplug them.
The solution is to turn them off for a while, or give them a good whack, and when they work again, use the headphone jack on the soundcard rather than the monitor/speaker system.
the headphones work when i plug them into the speaker jack but only the right one works (same as the speakers), but i found that if i plug it in half way both the left and right headphones work.
EDIT: not sure when it started, has been like it for a while i think
Plugging it in half way probably plays the right signal on both sides of the headphones, not really a good solution.
Have you checked the computer side of things, maybe try another sound system to check.