Hey 1949lea
How is your Leghorn going?
I am definitely not an expert at what ails chickens but as no-one else has replied, I will see if I can help.
Is she making frequent trips to the nest box at all or looking like she is straining to lay an egg? She may be egg bound.
I believe Leghorns are not prone to broodiness but she may be an exception.
Any chance she is coming into a moult? This could be the reason she has stopped laying and because she is not feeling 100%, picking on the other hens.
Is she showing any other symptoms of being unwell? Do her poops look normal?