This happened to one of our windows 2003 VMs this week - it was patched and rebooted and came up with no network traffic being passed.
The team first looked to revert to the previous known good state - the central storage snapshots.
But as it turns out these restored snapshots also came up with no network being passed.
I removed the old VM network adapter (AMD) and replaced it with the newer Intel, updated the vmwaretools, several reboots in between, all to no effect (the adapter was up and "connected") but not passing any traffic - could not ping the default gateway.
Finally found a post leading us to check the System Event logs
And finding a VMware KB article confirming the resolution:
Disable the Windows 2003 IPSec service and reboot - voila, traffic now unblocked!
Hope this saves other folks some time to resolution!