So my cat just ran away and I can't find her?

My parents are just awful, as some of you may know. My dad must have left the middle door open when the neighbor came in, so my cat probably just ran away at that second.

She was 3 when we adopter her and now like 5. She was a stray cat. There are so many stray cats where I live, I lost count. Sometimes my cat watches them from the window. Im always with my cat. I pet her, I kiss her, I cuddle and sleep with her every night. We feed her very well, she is a bit fat! I don’t understand why she ran away. Now she is out of sight. I looked everywhere! I’m terrified that she may be hurt. What’s going on? Did she go away to die or something? AND why is she running away when she has everything! And is she happier outside? Then I will be okay. I just want her to be happy. Please give me some insight.