Cannot register CS6 Master Collection product with valid serial number

[ReeceG]

I have a valid CS6 Master Collection licence and can see this when I sign-in to Adobe.  One of the programs/apps stopped working the other week so I reinstalled using the original DVDs.  Now I am unable to register the product.  The Product registration page tells me that the serial number is not valid.

[Govardhan.V]

Please refer below link :

https://helpx.adobe.com/ca/creative-suite/kb/serial-number-validation-launching-cs6.html

[Nancy OShea]

From whom did you purchase the software? 

Did you enter a 24 digit serial number?

Was this an upgrade or a full scale version?