That sounds like it is likely to be a formatting issue. I would highlight the problem cell, right click on it and choose Format Cells. Change it to a simple selection like General and see if the problem is still there. Also, to make sure it's formatting, look in the formula bar at the top to see if the actual value that you have entered is still in the cell. It sounds like what happened is that the cell data type got changed to text and then a custom format was applied of just 0. If you apply a custom format with some text in it, it will just show the text in the formatting rather than the value in the cell. If you are looking to have the cells show a default value of 0 until a value is entered, let me know and I can tell you how to do that.
There are no pop ups, if such pop ups arise that means the sheet is password protected or the cell may be validated via data validation. But no pop ups, it allows me to enter any value and the cell accepts the value but as soon as the cursor moves to another cell the value in the cell A19 hcanges to 0.
At this point, if it is the entire row that is a problem, I would suggest deleting the row and inserting a new one. Right click on the row heading (number on the left) and choose Delete. Then right click on the row heading that you want the row to be above, and choose insert. This should be a fail-safe way to get rid of the problem.