One of your physical network cards is down on your ESXi host. (I am using ESXi 5.5 in this example)
What are some command lines you can run in attempt the bring the VMNIC online?
If the vmnic is still offline, I check the switch port status and even check for a loose cable.
If you can reboot, go into the BIOS and see if it can see all 4 NIC…it could be the hardware died.