Calc multi page spreadsheet lost data

I’ve been using a calc sheet for accounts, saved as xlsx because I also access it from an excel only machine. This year when I cleared down the data to start a new year, I noticed in the first 2 or 3 sessions that sometimes when I opened it, some of the data on the main page was missing - it only showed 20 lines rather than 100. Opening it by starting calc & then using file open rather than using a recents shortcut seemed to solve it until today. I’ve some earlier backups, but don’t want to risk corrupting them & certainly don’t want to have to reconstruct all the data from scratch. I can try it in excel tomorrow, but that’s not really the point:( Any suggestions for what’s happening, or how to fix it?

Incidentally, this is the first time I’ve used this question page - I ended up typing part of the question about 5 times in the search or ask a question box before finding my way to this page. Perhaps it could be more obvious what to do?

Thanks

I spend days over weeks building 4,000 line spreadsheet and pivot. At 3am this morning I finally finished it, saved it and went to bed. I got up this morning and I only has 130 lines left and some columns had changed.

It will take me days to rebuild this, I simply cannot believe this.