LR 6 won't open - get mesage Catalogue named LR catelogue cannot be opened because another application already has it open

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installed LR6 from disc and it worked fine for 3 weeks. then it wouldn't open and was told to reinstall. have done that but now I get message 'catalogue named Lightroom catalogue cannot be opened because another application already has it open. have lost a day tring to resolve the issue.