First you need to know if the problem is truly the headset or the source. Try alternative audio ports if available.
Try the headset on another audio source to see if "stereo" returns. Best to also try another headset on the original audio source.
Determine if the problem follows the headset or stays with the audio source.
If headset then try the wiggle test. Gently push, pull, twist, and bend the wires to see if some movement results in sound returning to the left earpiece. Work your way along the wire to the plug(s). If any sound is briefly heard then there is most likely some break in the wire area being flexed.
If not then take a close look at the plug t be sure it is not damaged in some manner: bent, crushed, scratched.
Then move to the audio port being used. Check the port for debris, signs of damage, a loose connection inside, etc..
Could well be the headset is defective and, if so, hopefully returnable under warranty.