RSS Feed stopped working

My RSS ( http://www.theartofad.com/feed/ ) Feed has been working fine most of the time until now. I have noticed that sometime it shows and sometime it does not (does not show if I point my browser to feed url). Since several days ago it does not show at all – just blank page, no errors messages, just blank. I have not really changed anything, maybe added two posts, and added button on the side bar.

What could be wrong? Where can I start troubleshooting?

Thanks in advance for any help or advice.