Powering down and disconnecting the power cable won't cause iLO password reset. All the settings of iLO will be available even when power is removed from server.
If you are experiencing problems with password reset, you can use HP Online configuration utility from Operating system present on the server to set the IP/username/password and other values. Check in
www.hp.com/servers/lights-out to obtain the utility.
You need to have Administrator/root privilege to run this utility. And also you need to down sample XML scripts available to configure the settings.