If the software is still installed on an old computer, you can go to Help > System Info and copy the data to a text editor to locate the serial number entry.
If you lost your key, Adobe support may help locate it if you have proof of purchase. Adobe photoshop c6 serial number
If you restart Photoshop, a dialog should appear with two choices - Continue trial or Enter serial number. serial number of installed CS6? - Adobe Community If the software is still installed on an