Crank arm issue?

A crazy thing happened this morning during my morning ride. My left crank arm came off, during mid-stroke. I didn't crash or anything. It just took me by surprise. How do I fix this since it looks like there is a silver washer/spacer that is needed (see pics from my wife's Halfbike). The threads don't look stripped, so I'm at a loss at what causes this (and how to prevent it in the future.) Any suggestions from the people who've been riding for a while?

Hi Dean, I believe Denitsa has already contacted you and we'll be sending you a new crank set :)

Thanks Bo. Yes, Denitsa was super responsive and I'll get my crank set in a free days. Many thanks from the Halfbike Team!