The host that you provision with vSphere Auto Deploy cannot contact the TFTP server.
When you attempt to boot a host provisioned with vSphere Auto Deploy, the host performs a network boot and is assigned a DHCP address by the DHCP server, but the host cannot contact the TFTP server.
The TFTP server might have stopped running, or a firewall might block the TFTP port.
If you installed the WinAgents TFTP server, open the WinAgents TFTP management console and verify that the service is running. If the service is running, check the Windows firewall's inbound rules to make sure the TFTP port is not blocked. Turn off the firewall temporarily to see whether the firewall is the problem.
For all other TFTP servers, see the server documentation for debugging procedures.