Spreadsheet was save several hour prior. Can spreadsheet be recovered
I don’t know which machine and OS you use; I would try to shut down all and restart your PC.
If this doesn’t help please read: What information should I include in a question in addition to OS and LibreOffice version? - #4 by ROSt53
I am sure someone can help you.
this problem happened to me today, after I upgraded my version to 18.104.22.168, my OS is Ubuntu 14.04.