Check list

Is the PLC connected to the power supply
Are the participants available in the TCP/IP network
Can the PLC be reached via the PING command
Can the PLC be reached via TELNET
Are the PLC and the PLC connected with the right cable
Did you select the right COM port
Do the communication parameters match (Baud rate, parity, start/stop bits,...)
Is the COM port blocked by another application?
Did you configure the net address correctly, both in the driver dialog and in the address properties of the variable
Did you use the right object type for the variable?
Does the offset addressing of the variable match the one in the PLC?
Use the DiagViewer for further analysis -> Which messages does it show?