I'm hoping you had the computer turned off while you were plugging and unplugging your headphones from the different jacks.
Assuming you are using Windows XP try this:
1. Click Start/Control Panel/Sounds and Audio Devices
2. When the Sounds and Audio Devices window opens, make sure you are on the Volume tab.
3. Click the Advaced button under Speaker settings.
4. From the Speaker tab, select Stereo headphones from the Speaker setup: box.
5. Click the OK button...all the way out...until your back to the Control Panel.
6. Close the Control Panel.
7. Shut down the computer...and plug your headhones into the speaker jack of the sound card...or in the speaker jack (if you have one) on the front of your PC.
8. Turn the computer back on to test your sound.