iCloud ID and Password is the same as Apple ID and Password.
To possibly locate the phone on a map, go to www.icloud.com/find and enter the Apple ID and Password of the person who set up the phone originally.
Apple ID is the email address. If the Apple ID Password has been forgotten, click this link:
If you forgot your Apple ID password - Apple Support
If the battery is working OK and phone has a good Internet connection at its current location, the phone will show up on a map. If you see a message that the phone is "Offline", the battery has run down or the phone does not have an Internet connection, so it will not show up.
Unfortunately, the serial number will not help here.
If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support
Realistically, the phone is gone.....probably already on the way to China to be remanufactured.