1: Check your speakers (or speaker software if you have special spkrs like 5.1 or 7.1) for a balance adjustment and see if its working or centered.
2:Click on the speaker down by the clock and then select "Open Volume Control"
This will open the Master Volume Panel. Make sure your volume sliders are all the way up and the Balance sliders are perfectly centered.
Leave Master Volume open
Rt Click on the speaker down by the clock and then select 'Adjust audio properties"
From the volume tab click on 'Advanced" button in the Speaker Settings Section.
Make sure they are set to your type something like Desktop Stereo Speakers
Retest with headphones that you know work fine.
3: If that still only has one sided audio then:
Open the pc, remove the front panel audio connector from the motherbd, plug in your headphones to the rear and power on. If its still only playing from 1 side then I fear something on your motherbd has failed.