Problem editing cell in Sheets

Problem editing cell in Sheets

This may be a basic question: When I double click on a cell to edit its contents, the cursor moves to the end of the text toward the right side. I have encountered a cell that does not behave that way - when I double click on it, the text disappears and the cursor is on the left in what looks like a slightly larger cell covering the text .I am unable to edit the contents of the cell. What is causing this? 

Thanks.