Hey Chava, it probably got clogged with debris. I've cleaned a few out and I'll share the method I used to unclog the headphone jacks.
First thing's first you have to be extremely careful because at the bottom of the headphone jack, it's only protected by a thin strip of white plastic. If you break through that you'll have to replace your iPod altogether.
Get the smallest tweezers possible and try to get the debris out. Normally this doesn't work though, so I usually have to get a toothpick and CAREFULLY try to dislodge the debris and drag it out of the headphone jack. A slow process, but the end result is worth it.
When it's all clean, you can take a cotton swab and dampen it with isopropyl rubbing alcohol. Take care here as to not soak the cotton swab, but dampen it just a little. This will clean up any dust and/or dirt left on the sides of the headphone jack. Good luck!