Certified GKH Gardening Expert
Oh boy... That was a fungal infection caused by the Palmetto Weevil. It was likely attacked by the insect and taken within a very short period.
This extension will be able to tell you more about these insects, and how to control them: https://entomology.ifas.ufl.edu/creatures/orn/palmetto_weevil.htm
This article will help you to care for Bismarck Palms: https://www.gardeningknowhow.com/ornamental/trees/bismarck-palm/growing-bismarck-palms.htm