Just so you are aware this is a user to user forum, so the replies you get here are mainly from users like your self. You have provided very little info to go by and my suggestion below is just a guess.
You may have downloaded the LR 5 Creative Cloud trial version and trying to input the perpetual version license key, that will not work.
Try one of the links below to get the appropriate version.
Thankyou. Sorry no idea how to use forums ... not my scene. No I downloaded LR before Cloud was invented but it won;t let me reactivate. Will have to wait to talk to someone next week as getting nowhere fast.