This sounds like an examination question.
It could be due to many reasons why the patient was found unconscious but based on your report, it could be due to excess amount of oxygen given to a COPD patient with exacerbation who is already on type 2 respiratory failure
causing Co2 narcosis
. You didn't mention the pH of the blood gas report.
COPD patient have low oxygen in their system. Their brain is set to accept low level of oxygen and when high flow oxygen is given the brain loses it's ability to drive the respiration and patient becomes less responsive. Usually with a machine called BiPAP they respond quite well in such situation.
Patient could also have other events like stroke or a heart attack
(MI) leading to loss of consciousness
. I hope the patient responded to CPR and mechanical intubation