Cats tend to revert to being little wild animals when they're scared.
Put the litterbox outside, very near the house. Don't clean it out first - the smells will be familiar to her.
If she hasn't shown herself by tonight, get a humane trap, a good size one, and bait it nightly with a smelly food - tuna, etc. Put the trap under a bush or somewhere else where she would feel secure approaching.
One of my sister's cats got out this year, and it was six weeks before my sister caught her - she only caught a glimpse of her once in all that time.