How difficult is to sue thye airlines for lost luggage?

My wife flew from St. Louis to New York on Delta. The plane she had to change to in Detroit was so small they would not allow her to take computer bag on containing brand new laptop, camera, and jewelry. They redtagged it and sent it off to neverland and of course it bever showed up in New York. It says clearly on the website that laptops are not to be checked so I dont understand how they wouldnt allow her to take it on the plane. I believe they should be liable!