My kid asked why daddy cries and I didn't have an answer

My 6yo walked in on me having a rough moment yesterday morning and asked why I was sad. I made up something about being tired but the truth is I'm really struggling and I don't know how to even explain depression to a kindergartener. Let alone explain it to myself. I feel like I'm failing him by not having my shit together. How do you talk to your kids about mental health without worrying them?