Attached database to SQL Server 2016 and now won't work in SQL Server 2014
When you attached the mdf file(s) to the SQL Server 2016 instance the database was upgraded. The files were marked as being of the new version. There is no downgrade path in SQL Server.

The best option is restore from backup. Else, you many be able to script the schema and data and build a new database on the 2014 instance which is identical to the previous one. Here's one way of achieving this. Be aware of the things the tool may not script, like users, permissions, synonyms etc.

See also: How to move a database from SQL Server 2012 to SQL Server 2005