This problem can occur in the following situation:
Let us say there is an active Hyper Terminal connection with the USB-UART, for example at port COM18. The USBUART project is disconnected from the USB port without first disconnecting the connection in Hyperterminal. Now, if the USBUART project is plugged into the same USB port, it will not be recognized.
In order to avoid this situation, before unplugging the USBUART from the PC, disconnect the connection using the Disconnect button in Hyper terminal (the small icon with a telephone). To connect it, plug in the USBUART to the PC, and connect using the Connect button in Hyper terminal.