Endnote X7 suddenly not opening on Mac

You can try the following steps to Recover the Library:

https://support.clarivate.com/Endnote/s/article/EndNote-The-proper-process-for-recovering-library-files?language=en_US

If that fails then Please contact Technical Support directly

1-800-336-4474 option 4 then 1 or

https://support.clarivate.com/Endnote/s/technicalsupport?language=en_US