Well, she either jumped out and fell somewhere behind the tank (happened to me once with one of my Endlers), or she's hiding somewhere under the decorations, or (worst case scenario) she died and is lodged somewhere out of sight. If she died a while ago then she could've decomposed or been eaten by the other fish, so you wouldn't be able to find her. Not sure how anyone here can help, we've never seen your tank in person
You'll just have to look for her, and maybe test the water just to be safe (dead fish can quickly foul the water). She could also be pregnant, females tend to hide in a secluded spot to give birth. I really don't know, there are many options... When did you last see her?