Run RWEverything which will check your hardware for the product key. Select ACPI tables and the select the MSDM tab: [URL] If there is no MSDM tab there is no product key.
I have a dell inspiron 15r 5520. lately the hard drive failed and so i replaced with a new 500gb one, installed windows 8.1 from the internet and now i don't know where to find my original product key to activate windows. Any help please. 
No Product key