it's due to the intubation. they likely roughed up your vocal chords and it's irritated. i had no voice for almost 2 months and they were going to do a procedure to inject fluid into the paralyzed chord so the other side met it. while i was scheduling this (over a week and a half) my voice started to come back little by little. i did the vocal exercises (saying and repeating the words: Up, Elephant, Elevator, Inside, etc..) i was speaking fully about 2 weeks after it started to come back.
What you describe is apparently pretty common. Usually, the voice returns by itself after a few weeks or months. If it does not, voice therapy (basically physical therapy for the voice) can help. Here's more information:
http://www.chdbabies.com/2009/08/vocal-cord-paresis-can-occur-after-open.html
Similarly, I had triple bypass 4 weeks ago and my voice has been terribly affected since. Is this permanent if not what is the treatment?