Cells not responding to left, right, center arrangment

[amsanborn]

I think I did something but I don't know What. I select a cell that has a 20 font Size I enter a number it show at the 20 Font. Now when I go some ware else in the work unselecting that cell the number went to font 10 and droped to the left bottom of the cell. When I select the cell again and select the the left middle and right to move the number it does not responed. It happens everyware in the worksheet.