What model PC and Operating System? (that should be on any initial problem post - it can make a difference and allow someone to give you specific assistance).
General suggestions (1) verify that the headphones are good. (2), make sure you are fully inserting the headphone plug into the speaker jack. If you are still having problems, power off the PC, open the case and reseat (unplug then plug back in) the connector from the front panel headphone jack to either the motherboard (if you have on-board sound such as SoundMax) or to the SoundBlaster card.
There's a possibility that the headphone jack is damaged and not making good contact, but that is not very common.
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