Hyper-V starts and then immediately stops (goes back off)

Hello Shabin, I am Henry and I want to share my insight about your issue.

Since the VM was working before and the VHDX files seem intact, the problem is likely related to the guest OS boot configuration, a host resource issue, or a corrupted VM configuration file.

You can refer a step-by-step troubleshooting guide:

Step 1: Check the Hyper-V Event Logs

• On the Server 2019 Hyper-V host, open Event Viewer.
• Navigate to: Applications and Services Logs > Microsoft > Windows > Hyper-V-VMMS > Admin.
• Look for Error or Critical events that occurred at the exact time you tried to start the VM.
• Common errors you might see include messages about resource allocation (RAM, CPU), storage path issues, or other failures. The event details will guide your next steps.

You can take a screenshot or export the log file—I’ll help you review it. Please let me know when the issue occurred. A specific timestamp would be ideal, but an approximate time is fine too.

Step 2: Troubleshoot Guest OS Boot Failure

1. Boot into Safe Mode:
   • Start the VM and immediately open the console window.
   • Repeatedly press the F8 key as it starts. This should bring up the "Advanced Boot Options" menu for the guest OS.
   • Select Safe Mode and press Enter.
   • If the VM starts and stays running in Safe Mode, the problem is a recently installed driver, Windows Update, or a service within the guest OS. You can then use tools inside the guest (like Event Viewer, or uninstalling recent updates/drivers) to fix it.
2. Repair the Boot Configuration Data (BCD):
   • The bootloader inside the VM may be corrupted.
   • Mount a Windows Server ISO: In the VM's settings, connect a Windows Server 2019 (or matching OS) ISO file to the virtual DVD drive.
   • Boot from ISO: Start the VM and boot from the DVD drive.
   • On the Windows Setup screen, click "Repair your computer" > "Troubleshoot" > "Command Prompt".
   • In the command prompt, run the following commands in order:
     • bootrec /fixmbr
     • bootrec /fixboot
     • bootrec /rebuildbcd
   • After they complete, shut down the VM, disconnect the ISO, and try to start it normally.

Step 3: If the guest OS seems fine, the problem could be with the host or the VM's configuration.

1. Check Memory Allocation:
   • In the VM's settings, go to the Memory tab.
   • If you are using Dynamic Memory, try disabling it for testing. Set a fixed (static) amount of RAM (e.g., 4096 MB).
   • Sometimes the host doesn't have enough available memory to satisfy the VM's "Startup RAM" requirement, causing it to fail immediately.
2. Re-create the VM Configuration File:
   • The VM's XML configuration file might be corrupted. You can fix this by creating a new VM and attaching the existing virtual disk.
   • Important: Note down the VM's settings (RAM, vCPUs, network switch, etc.).
   • Create a New Virtual Machine in Hyper-V Manager.
   • Configure it with the same settings as the old one.
   • When you get to the "Connect Virtual Hard Disk" step, choose "Use an existing virtual hard disk" and browse to your VHDX file.
   • Finish the wizard and try to start this new VM. If it works, the old configuration was the problem, and you can safely delete the old VM object.
3. Check Antivirus Exclusions:
   • Aggressive antivirus software on the Hyper-V host can sometimes interfere with VM files, causing startup failures.
   • Ensure your host's antivirus has exclusions for the default Hyper-V folders, processes, and file types (.vhdx, .vhd, .avhdx, .xml).

Hope one of these works for you.