I bought a physical copy of CS5 in 2011 and I still have the order number, with detailt. The box with the CD and serial number was lost/stolen. What can I do? Do I have to buy a new one?
Did you register it? If you did, the serial number can be retrieved by logging into your Adobe user account.
If you still have the application installed and running on one machine, but you didn't register it - try to do that now. The serial is automatically transmitted in the registration process.
If the software was stolen there's a good chance the serial number has already been put to "good" use, and you'll probably be unable to activate it. In that case you should contact Adobe.
And not to rub it in, but hopefully for the benefit of others out there reading this: It's the serial number you pay for. Keep it safe. The software itself is free and worth nothing.