See these docs, particularly:
... if you shut down half or more of the master-eligible nodes all at the same time then the cluster will normally become unavailable. If this happens then you can bring the cluster back online by starting the removed nodes again.
You had two master-eligible nodes, and shut down one of them, which is "half or more", so this paragraph applies. The only safe way to proceed is to start the lost node again. If you no longer have this node then this cluster is dead, you'll need to start a new cluster and restore the data from a recent snapshot.
The lost node held the vitally-important cluster metadata, without which the data on your data node is meaningless.