The story required Jessica to accidentally let go of the rope, but I was at first uncertain how to express this idea in Latin. Here is a little of the process that helped me arrive at an (I think) acceptable solution. I was comfortable with accidentally, which is commonly expressed with casu, the Ablative of casus. The historian Nepos wrote casu accidere to happen by accident and Tacitus wrote casu procidere to accidentally fall forwards. The Noun casus itself means a falling down, and by extension an accident. So I guess the Ablative casu might be considered an Ablative of Manner. How did it happen? It happened casu by accident, accidentally.
The expression to let go of took a little more work. It is, for starters, an expression that has a wide range of uses in English. You can let go of an animal that has been caught in a trap, let go of an idea that has been keeping you up nights, or let go of a rope that you are descending from a rooftop. I needed a phrase that expressed the last. Scouring dictionaries for various uses, it seemed that the most appropriate Verb would be resolvere to unbind, untie, loosen, open, release, free and that the full expression would be Jessica released the rope from her hands.
This was the bulk of the work done. The rope funis would be the Object of the Verb and would therefore be written in the Accusative Form, funem. And as Jessica released the rope from her hands, an Ablative would be required, manibus. Grammarians call this an Ablative of Separation. This left me with the final expression, funem manibus casu resolvit. All that now remained was to run the expression by a couple of people whom I much admire for their Latinitas Latin style, Latinity. I thank them for their help, and am happy to say that it passed their scrutiny.
Jessica, shocked by the unexpected shout, accidentally let go of the rope and fell to the ground. The stolen suitcase was lying beside her.