I had this problem when I was in a very humid location (Thailand) for a while. My phone wouldn't charge and USB to PC stopped working too. Just something to consider.

If you think it could be humidity, stick the phone in a sealed bag of uncooked rice for 24 hours or more... or do as I did and suck air through the USB port of the phone. 18 months since I had the problem and I haven't had a recurrence.