How to troubleshoot a missing index or constraint ?

If you created some constraint ( CREATE CONSTRAINTsee details here) or index ( CREATE INDEX- see details here)  on nodes or relationship ( or legacy fulltext index via  db.index.fulltext.createNodeIndex or db.index.fulltext.createRelationshipIndex) and subsequently receive an error stating the index or constraint is missing when making a Cypher query or any API call that would rely on that same index / constraint please check the following:

• Check the index or constraint was successfully created withSHOW INDEXES( see details here) 
•  Review the log file query.log for any index or constraint drop (drop index, drop constraint)
• Review your queries and check any calls to CALL apoc.schema.assert(...).
  The default behaviour is that when calling
  apoc.schema.assert({indexLabel:[indexKeys],…​},{constraintLabel:[constraintKeys],…​}, dropExisting : true) yield label, key, unique, action
  the last parameter dropExisting is by default true and therefore the schema is replaced.
  See further details here.
• Check that no database dump or database export has been loaded that would overwrite the existing "schema".

If you tried everything above, regularly poll on SHOW INDEXESand log the output to a debug file in order to narrow down the time interval of the issue and review the queries or events.