I just downloaded the latest version of Libre Office - version 5.1, I think, but it will not open my older version of Libre Calc documents. What should I do

I had the version 4 of Libre Office and have created many Calc documents with that version. I just upgraded/downloaded the latest version of Libre Office/Calc (version 5.1 I think) and it will not open my older version Calc documents. WHat should I do? I can’t figure this out…help!