After all nodes of the HA cluster went down, StarWind VSAN may not able to determine which one holds the most recent data (i.e., was powered off the last). In order to prevent possible data loss (i.e., when a device with actual data is overwritten), StarWind VSAN blocks all the incoming connections until the synchronization source is identified and synchronization begins.
If you face the “Not Synchronized” device status on all StarWind nodes, you would need to discover which node has the most recent data. To do so, investigate Application logs on all nodes and identify the node with the synchronized device(s) at the moment of an outage.
If you are not sure which node has the actual data, contact StarWind Support or stop and disable StarWind VSAN service on all nodes except the one you assume to have the most recent data, mark the devices on that node as Synchronized, and check data consistency. If the data is not relevant (e.g.., old data appear, VMs are not able to boot, etc), stop the service on the current node, run it on the next one, Mark device(s) as Synchronized, and check data consistency on it. If you succeeded in determining data consistency, simply run the service on the nodes where it is not running yet and let the devices get synchronized.