Contact the company.... (If you registered it)... I had the same thing happen with my Pro Tools LE...
If not... The Store where you bought it will have the serial number... Take your receipt and have them look it up (Sometimes there is a fee for this)..
If YOU don't have your reciept, go to the store and tell them you bought it from them and lost your reciept... It is simple as giving them your name and your phone number for them to dig up the invoice.......
Don't let them tell you they can't pull it up... By law a business has to keep the invoices for 6 or 7 years.....In case they get audited...
If you bought it used... Well you'll have to go through the person you bought it from..
As luck would have it, I found your lost serial number. It's 876YYH09765JJJ0435989879LKLKLL078965123456DORK.
After you enter it in the required field, you need to stand up and turn around three times in place while chanting Cakewalk, Cakewalk, Cakewalk, Cakewalk. If that doesn't work, you are not turning fast enough. Try it again, but turn faster.