Create a new library and save it to your desktop (unless you have a Mac and iCloud syncing of your desktop).
Run a search for duplicates in your EndNote library. Remove duplicates if there are any.
Try adding references manually. If you do not have any references to add, test with the followings:
Create a couple of groups and group sets and move some of your references into them.
Find full text
Add PDF attachments manually
Import PDF files
Create a compressed library and save it to your desktop.