If the disk has not been damaged, it is more than likely that your CD/DVD burner has died.  I've had this happen several times. 

Do you have access to another system with a working CD/DVD to test the disk.

DiskInternals sell a CD/DVD recovery app.  The site is DiskInternals.com.

Unlike CL and EF, SMS does not have the option of purchasing a back up disk.  I wish they did.  I used to buy both the extended download and a back up disk just to be on the safe side.

Since you purchased a physical copy, they may give you a download link or they may require you to purchase their extended download option.  It gives you unlimited downloads for up to 2 years and costs less than $10 USD.