files loading with missing data in Numbers

Recently, I added data to a Numbers spreadsheet. Saved and closed. A few weeks passed and the data was missing. It was recent enough that I remembered the data and re entered it. It happened again. Still thinking it was my error but now I am aware. Today, I loaded a Numbers file from last year. Half the data was missing from the cells. I have only noticed this with Numbers files. Of course this is a problem because I need to the old file on occasion for reference. What is happening?