No Volume Exist - 4 disks full

Hello Community, 

I realized today that I did not have access to my NAS anymore. None of my files are accessible. 

I have a RN104 with 4 WDRed 3To disks. Last time I checked, two months ago, they were healthy. 

I see them completely full today and I see the following error messages. 

As you can guess, my NAS has all my backups and data... I cannot imagine losing their content...

What could I do to try and restore the system? 

Here are the logs:

Apr 23, 2021 05:50:52 PM	System: ReadyNASOS background service started.
Apr 23, 2021 05:47:05 PM	System: The system is rebooting.
Apr 23, 2021 05:27:38 PM	Disk: Disk in channel 4 (Internal) changed state from RESYNC to ONLINE.
Apr 23, 2021 05:27:37 PM	Disk: Disk in channel 3 (Internal) changed state from RESYNC to ONLINE.
Apr 23, 2021 05:27:36 PM	System: Alert message failed to send.
Apr 23, 2021 05:27:36 PM	Disk: Disk in channel 2 (Internal) changed state from RESYNC to ONLINE.
Apr 23, 2021 05:25:38 PM	System: ReadyNASOS background service started.
Apr 23, 2021 05:25:11 PM	System: ReadyNASOS service or process was restarted.
Mar 05, 2021 01:20:43 PM	Disk: Disk in channel 2 (Internal) changed state from ONLINE to FAILED.
Mar 05, 2021 01:20:21 PM	Volume: Volume data health changed from Redundant to Degraded.
Mar 05, 2021 02:55:04 AM	System: A hotfix was applied to your system (6.10.4 Hotfix 1).
Mar 04, 2021 02:52:05 AM	System: ReadyNASOS background service started.
Mar 04, 2021 02:51:41 AM	System: Alert message failed to send.
Mar 04, 2021 02:51:41 AM	Volume: Less than 10% of volume data's capacity is free. data's performance is degraded and you risk running out of usable space. To improve performance and stability, you must add capacity or make free space.
Mar 04, 2021 02:51:39 AM	System: ReadyNASOS service or process was restarted.
Mar 04, 2021 02:41:18 AM	System: ReadyNASOS background service started.
Mar 04, 2021 02:40:54 AM	System: Alert message failed to send.
Mar 04, 2021 02:40:54 AM	Volume: Less than 10% of volume data's capacity is free. data's performance is degraded and you risk running out of usable space. To improve performance and stability, you must add capacity or make free space.
Mar 04, 2021 02:40:52 AM	System: ReadyNASOS service or process was restarted.

Thanks in advance for your tips,