Because all the instantly recovered virtual machine (IR VM) read requests ultimately come from the Backup Exec media server storage, if the media server becomes unavailable for any reason, then the IR VM will not be able to read from its virtual disk.

As soon as the media server restart is complete, the IR VM is brought back online automatically. However, from the guest OS perspective, depending on what the guest OS and/or an application inside the guest OS is doing, they may not be able to well handle unexpected unplugging and replugging its hard drive. That causes Windows to hang.

If Windows hangs after restarting the Backup Exec media server, try to power off and on the IR VM from the hypervisor host.

If a media server outage is planned, the IR VM can either be shut down or suspended prior to the outage. Once the media server is available, The IR VM can be powered on without losing any data saved prior to shut down or suspend.

For more information, please refer to the following blog posts:

Enhanced Instant Recovery resiliency for VMware in Backup Exec 16 FP 1
https://vox.veritas.com/t5/Backup-Exec/Enhanced-Instant-Recovery-resiliency-for-VMware-in-Backup-Exec/ba-p/831238

Enhancing resiliency for Instant Recovery
https://vox.veritas.com/t5/Backup-Exec/Enhancing-resiliency-for-Instant-Recovery/ba-p/829649

Windows inside the instantly recovered virtual machine may hang after restarting Backup Exec server.