I believe you would need to confirm the power-off time for both servers first. You should see it in the Windows System logs on both servers. Right before that event make sure to search for any events regarding the stop of the StarWind VSAN service. In the Windows Application logs you will see some messages for non-synchronized state of the devices. Based on that information you can decide on which node to mark as synchronized.
If your case is a hard one with someone just performing a hard power-off of the servers, in the Windows System logs you will see event ID 6008 (improper shutdown). Notice the time indicated in that event for both servers. After that, compare the time settings for the two servers and take into account any time different between them. Based on this information, select the node to mark as synchronized. Before doing so, I would recommend stopping and disabling the StarWind service on the other node(s) in the setup not to trigger automatic sync. This is done to ensure data integrity even if you mark the wrong node as synchronized. After you confirm the correct node was selected and marked, go ahead and start the partner's StarWind VSAN service.
For StarWind VSAN Free, you would need to use the script called SyncHaDevice.ps1 and select the proper action to perform. If you are to mark some certain device as synchronized, make sure to uncomment
and comment out the preceding line.
Hope this helps, but feel free to ask for any clarifications.