If you do use server -jc -c (aka Domino starts as Java controller mode -> add <TCPIP>_TcpipControlerAddress=...... to add the IP address for port 2050.
If you do not user Domino controller, check was is starting in servertasks=.... in your Domino ini file and also check in windows service information which account you do use to start Domino (LocalSystem or other) if you use a domain account, check password and account rights, if using local account (not local system), check if it is ok and the password was not changed