What do you see at Enclosure level?.
Check the setting at vCenter server for respective switch , if network configuration is correct.
Is this impacting all virtual machines or subset of virtual machines?
If you migrate VM to another host using vMotion or Cold Migration, does the issue persist?
Is there anything in common between the VMs with issue like any specific VLAN?
Are the virtual machines able to communicate with each other on the same vSwitch and Port Group? Or is this a complete loss of network connectivity to the troubled VMs?