When the Fusion Drive is crashed and the metadata is damaged, you can't boot your Mac from it and access your files. According to your description, you failed to mount the Fusion Drive in Sigle User Mode.
The only way to recover data from a corrupted Fusion Drive is to try third-party data recovery software if there are no backups for the Fusion Drive. Professional data recovery software such as iBoysoft Data Recovery for Mac can scan the unbootable Fusion Drive in macOS Recovery mode even if it is split into SSD and HDD for deleted and lost data.
Related read: How to Recover Data From Fusion Drive on iMac/Mac Mini?
After recovering data from the Fusion Drive, you can First Aid Fusion Drive in recovery mode and reformat the drive to make it usable again. In case the drive is split, follow Apple's guide to fix a split Fusion Drive.
