You're definately not an idiot! It could have happened to anyone.

You might want to put some canned food out since that is stinky and may lure her out to eat. Also, maybe you can set up another hiding place. That way if she comes out to eat, she may crawl into the new hiding place - maybe a box with a small hole in it or something similar.

Is there a feral cat rescue where you live? That may be better than the shelter for her.