If you're upgrading from VMware ESXi 5.1 to 5.5 on an IBM HS23 blade and getting packet loss or loss of all network connectivity, this is likely some strange issue with the elxnet driver for the Emulex network adapters. We've had success reverting to the be2net driver. I'm not sure if the stock ESXi 5.5 image will include the be2net driver, so you'll need to install the vib if this is the case. Once done, you'll need to manually enable this as the preferred driver.
1) SSH to your host or enable ESXi shell and Alt+F1 to bring up the shell login
2) As root, run the command esxcli system module set --enabled=false --module=elxnet and then esxcli system module set --enabled=true --module=be2net
3) Reboot your host