although opened it before.
Something might be corrupted. If XLSX file doesn’t hold sensitive or personal data, you should report your problem to LO’s Bugzilla (e-mail might be publicly visible) and attach that XLSX file. If you can’t due to some sensitive data in the sheet, try to recreate the problem on dummy sheet and attach it there so developers can take closer look. You do not mention LO version or operating system you’re using. That information may be important.