If you reinstall the Lightroom through the Creative Cloud desktop app, the one you used to install Photoshop, it should activate.
Uninstall it first, through control panel, if you are using Windows, or through the proper uninstaller, if you are using Mac.
Thanks Luis. I thought that too, so I deleted LR and then went to the App and it's thinks it's still there so can't reinstall it. What a pain!
OK, Googled how to uninstall LR, deleted a heap of files, logged into Cloud and there you go! LR gone and I'm reinstalling. Thanks for your help.