Some thoughts:
- is the VM connected to the correct port group
- is the virtual network adapter set to "Connect at Power On"
- are all physical uplink ports (which are in use for the vSwitch) configured properly (VLAN settings, access/trunk mode, port security, ...)
- does the guest OS show the network adapter as "connected"
- did you double check the guest's IP settings
- are you able to ping e.g. the default gateway, or another VM from within the guest OS
André