downloaded and reinstalled LR 5, bought LR2 and since then always updated, cloud says serial number doesn't work. How can i return to use it????lr proble
You probably downloaded the Lightroom5 Creative Cloud version, the Perpetual License key will not work. Uninstall this version (your catalog, preferences and personal files will not be affected) then download LR 5.6 from the link below and proceed to install, your license key should activate just fine, you will need both the upgrade key for LR 5 and your License key for Lightroom 2.
If you purchased the Photoshop Creative Cloud Photographers package then you do not need serial numbers to activate you have to download via the Adobe CC Application Manager.