I have 2 licences of Adobe Lightroom 5, which I had installed in one PC and one iMac.
Recently I had to make a clean instal of booth Operative Systems and I didn't uninstall Lightroom.
Now, when I try to validade the software licence in the reinstalled Lightrooms I receive a message saying that the serial number is not valid.
Can someone tell me what should I do to be able to use my licenses again?
You should contact Adobe support and have them work with you to restore things as necessary.
Serial number and activation chat support (non-CC)