Avatar will not play on my BDP-S300 Sony blu ray. Any recommendations on how to fix this?

It needs the 5.3 software update. You can download it if you have a PC or be SOL if you have a Mac, in which case it will take 2 weeks for Sony to get it to you by mail.

I was able to fix mine by doing a factory reset on the box. Avatar played one time but hung up part way through. After that it wouldn't play again including another copy of the blueray disk. Once I reset the box it played them both properly again.

I recommend you go to the Sony website and find if there are any updates for your blu ray player. If there are they will have you download the update, unzip the download to a new file, copy the file to your DVD burner, and install the DVD Update into your player, press "play" OK a couple of things and shazam! Updated! That is unless your lens just needed cleaned, Good Luck.