In order to help can you please provide the version of Illustrator and what operating system that you are using? Please also report as to what you were doing when the problem occurred. Does it always happen during a particular operation? Or does it occur randomly? Please give details.
Illustrator is fully up to date v 26.3.1. Windows 11. When I restart the
program, the libraries are available on the first file I work on. However
when I open any new file, the libraries are no longer available.
Sorry to hear about this experience. I would request to try the steps shared below and share your observations:
Looking forward to hearing from you.