Your license allows you to have two activated installations so if you only ever had the one there is nothing to prevent the second from happening. You can contact Adobe Support thru chat and ask them to reset your activations so that the lost one becomes available again, but this will reset all, so it is best to do that ahead of activating the new installation.
Chat is not available on weekends. For the link below click the Still Need Help? option in the blue area at the bottom and choose the chat option...
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Serial number and activation chat support (non-CC)
https://helpx.adobe.com/contact.html?step=PHSP-PHXS_downloading-installing-setting-up_lice nsing-activation_stillNeedHelp