If a source node is rebooted, and it can find its destination, replication will resume. This can be verified in the webGUI at STATUS-->Tasks. Email are also sent when the replication is interrupted, as well as the various steps until synchronization.

If the destination node is rebooted alone, replication will resume when it is back online.

If for some reason both machines are rebooted together (not recommended) the volumes will be marked as destination on each side in order to protect data integrity due to possible split-brain scenario between nodes. This was similar to your case as the last one that was standing lost power and when they came back online volumes where set to destination as we dont want start failover causing a possible split brain.

In a failover configuration, if the source node is rebooted before the destination, this will cause a failover event and will require human intervention to move resources back to the original source node.