Using a spreadsheet with numbers and 19 rows have disappeared

If it is the top rows (other than header rows) then your table may have been accidentally pulled partway off the page.

1. Select the table
2. Click and hold on the circle/bullet at the top left of the table
3. Drag the table back onto the page.

Or it could be as simple as the page is scrolled and the top of the table is outside of the window.

Also, make sure you have not hidden the rows by accident. Click in the table and go to the Table menu. Select 'Unhide All Rows' if that option is active.

SG