Hi Jackito, if you go to "About This Mac" found under the Apple logo in the top left hand of your screen and click on the version number twice it will tell you the serial number. Apple should have put back in the original number, which you will find on the back of your computer somewhere.
This attachment shows how there is no longer a serial number on my computer and Apple say it will have to go back for a 2 days to fix this. That would be the 3rd repair in 3 months...
It doesn't sound as if this would necessarily be the cause of the problem given that others are having the same issues but I was certain that sooner or later this would become a pain.