If a query is cached and its connection is subsequently rendered invalid (say, by inputting an improper db user password in the connection's settings) the query can't be re-run from cache. Per my understanding, a cached query is associated with a unique hash, which is a conglomerate of various parameters (connection, model, explore, etc). When a query is run, its hash is compared to the hashes of cached queries. If the hashes don't match up, Looker runs the query against the external db. Rendering the connection invalid by altering its settings may change subsequent query hashes and prevent them from matching up with the hashes of cached queries.