It would take some more information.
I understand that this is not a new installation but it occurred during the restart of the Esxi host ... correct?
Where did you install Esxi, on a dedicated SSD or on a large disk?
Here are a couple of links with your error, see if you can solve the solutions.
even though I'm not from 5.1 (it's very old as a version).
- Cannot boot -> ESXi 6.5 NOT_IMPLEMENTED bora/vmkernel/core/bootModule.c:492
- ESXi 6.0 PSOD "NOT_IMPLEMENTED bora/vmkernel/core/bootModule.c:112"
- [SOLVED] VMware 5.5 fails PSOD - Spiceworks
P.S. I try to understand how I can help you and I am writing to you again.
...in addition to the previous answer.
It is better to install a new Esxi, hook Storage \ Nas and register the virtual machines in the new installation.
Virtual machines are not lost because they are located in their folders within the storage space (do not format it).
If you can install ESXi on an SSD card just for this purpose.
If you have a backup of the network configuration, you can perform a configuration restore (I suppose you know it) if you do not have a backup, I recommend for the next time to always make a backup of the final configuration of your environment.