Problem with Adobe Photoshop CS2?

Whenever i open Photoshop, I get this message: "Your adobe photoshop username, organization or serial number is missing or invailid. The application cannot continue and must now exit" and after I click ''ok'', photoshop closes. I didn't have this problem before, i used to use photoshop normally. I don't know why this is happening now. Can anyone help me to fix this