A couple of things...
1. Are you sure the original text is gone or is it just not viewable due to
the cell not being big enough.
a. Try resizing the cell - both vertical and horizontal
b. Try turning on word wrap in the cell (Format/Cells/Alignment Tab) -
or even Shrink to Fit
2. If when you add text to the cell, is the entire cell activated
(typically blue) just before you start entering text? If so, as soon as you
type your first character, everything that was originally in that cell is
overwritten. A way to prevent this is to click on the cell, if the entire
cell is blue, click inside the cell again and you should just see your
cursor blinking inside the cell and the cell is no longer blue. Now you can
use your arrow keys to move around in the cell to the point where you want
to add your text. (Tip: if you want to add a carridge return (or multiple
lines) inside a cell, use "Alt" "Enter" key combination)