I'm currently running Workstation Pro 16.2.3 but was getting the same problem when using 16.2.1 and possibly early versions (it's been going on for 4 months now and I tried several versions during testing).
Essentially lots of my VMs, regardless of windows version installed (they're all Windows based VMs), have been failing to boot. When they haven't found the boot disk they've attempted a network boot before failing completely saying "operating system not found" or something similar depending on the windows version installed.
This is now affecting multiple VMs and I am unable to work. I thought it was a Windows update and have even tried a completely separate laptop. I've even taken clean copies of the VMs and had the same thing happen, so it seems to be Windows/ VMware issue but cannot pinpoint where.
I'm at a loss as to what else I can do and am desperate for any assistance anyone can offer.
"Operating System not found" simply means that the vmdk or iso that is currently assigned as boot-device is either corrupt or simply not bootable.
To fix that enter the BIOS/Firmware and check which device is assigned.