If mongos loses network contact with all nodes from the CSRS config server set (both primary and secondaries), the replica set monitor will deem this set as as 'unusable' and will stop monitoring it.

From this point onward all operations which need to access some config server metadata will begin failing with the following error:

> db.foo.find().itcount();

2016-03-16T16:59:20.941-0400 E QUERY    [thread1] Error: error: {
        "code" : 71,
        "ok" : 0,
        "errmsg" : "None of the hosts for replica set test-configRS could be contacted."
} :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
DBCommandCursor@src/mongo/shell/query.js:694:1
DBQuery.prototype._exec@src/mongo/shell/query.js:118:28
DBQuery.prototype.hasNext@src/mongo/shell/query.js:281:5
DBQuery.prototype.itcount@src/mongo/shell/query.js:407:12
@(shell):1:16

This includes the refresh of the list of shards, which needs to be read from the config server metadata. Therefore, currently there is no procedure to restart or retry monitoring of the config server set and the only recourse is to restart mongos.