Mongo DB sharded cluster fails when one shard is down
You have understood concept cluster and sharding wrong or you are mixing it with ideas of replica set. If cluster misses part of it (shard(s)), it is not whole and cannot function. 

Replica set can function when minority of nodes are missing and that's why (sharded) cluster is normally made from replica sets. So, that every shard in the cluster is replica set, at least three nodes. So, in two shard cluster, you have actually six data nodes (2x3) and top of that you have three node config server replica set.