How can I fix "serial number unavailable" issue?

Here is my product details:

MacBook Air 13'' Mid 2012

• 2 GHz Intel Core i7
• 8 GB 1600 MHz DDR3
• Intel HD Graphics 4000 1024 MB

Lately, I noticed that my iMessage could not be used. I also noticed that my MacBook Air Serial Number says "Unavailable". It seems to be a problem that happened without my knowledge. I did not replace or change any hardware parts. I did reinstall OS few times since I bought the MacBook.

Any advice would be great.

Thanks!