Use DSPJRN to list journal code ‘T’ and entry type ‘DO’ for the time period that the library was deleted. Review the entries to find the entry for that library. (The DSPAUDJRNE command might be easier for you, but it only outputs to the screen or to a spooled file.)
If you are not auditing for deleted objects or if you cannot obtain the QAUDJRN journal receivers for the necessary time frame, then it will only be sheer luck if you can find what happened. For example, you might look through every joblog on the system and find one that shows the deletion of the library; but that’s pretty unlikely.