If the original Hyper-V host computer was crashed, the best way should be restore the latest backup of Hyper-V.

However, if you don’t have a backup for the Hyper-V, here are the steps:

Scenario 1. No snapshots with the VM on the crashed Hyper-V computer

1 Create a new VM on the rebuilt Hyper-V server

2 Mount the existing VHDX to the VM.

Scenario 2.  There are snapshots with VM on the crashed Hyper-V computer

1 Merge the snapshots into the original VHDX first.

2 Repeat the steps in Scenario 1.

1 Open Hyper-V Manager

2 Click Edit Disk to open the Edit Virtual Hard Disk Wizard. Then click Next.

3 Click Browse to select the latest .avhd(x) file.

4 Click Next.

5 select Merge to merge the changes stored in a differencing disk into the parent or another disk. Then click Next.

6 click next and select To the parent virtual hard disk and click Finish.

Repeat above steps for each .avhd(x) file until all changes have been merged successfully into the parent virtual disk.