Dear Julia,
Have you tried using the recovery command under the tools menu in EndNote? Also, where is the library located? If it’s in a cloud folder such as iCloud, Google Drive or Dropbox, that will corrupt the library. So you would need to move the library’s .data folder and .enl file to your local machine before running the recover library command in EndNote/Tools.
You are welcome to give us a call here at support, 1-800-336-4474, press option 4, then option 1.