microphone isn't working

I have 4 headphones with a microphone and speaker that plug directly into my
sound card (one at a time, of course). I was using one of the headphones,
which worked fine, both input and output, for a long time, when the
microphone just stopped working. However, the speaker still worked fine.
Another headphone had this same problem. However, two other headphones
worked fine, both input and output. The problem is specific to my computer,
because the two "bad" headphones worked fine on another computer. Any
thoughts?