My spec page on my HP Pavilion HPE-112y desktop indicates 8GB installed Ram
Solution 1:
Enter the BIOS settings when the machine boots (by pressing F10/F2) and see how many (and what size) DIMMs the BIOS thinks are installed.
If the BIOS agrees with the OS and states that only 8GB is installed, but you're confident that 16GB should be fitted, it may be worth opening the case and checking that a DIMM hasn't been unseated in transit.
Obviously, if opening the case will void the warranty you may wish to follow John's advice and contact the vendor you bought the machine from.