A number of things you need to take into account,
1) The computer you initially configured the device on is the same computer (or at least has the same settings file)
2) If you are using a USB to serial converter, then you need to make sure that the COM port number has not changed. As if you plugged it into a different USB port the COM port number may change. Can you check in device manager to see what the COM port number is?
Check this first and we will take it further from then.