2011 Excel for Mac: Numbers will not display in cell

Please help. I am using 2011 Excel for Mac. Whenever I type in a number and press enter, the number disappears from the cell. The number is visible in the formula bar, but not visible in the cell. If I type a letter before a number, then the letter and number will be visible in the cell after I press enter If I spell out the number, then type the numerals, it will all be visible. But if I just type a number, it is not visible in the cell when I press enter; its only visible in the formula bar.

Why is it doing this?
Is there a setting I need to adjust?
Or is this a glitch in the program?

Thank you.

this may sound crazy but check your font color, i have done this to co-workers before. If your font color is set to white then you will see it in the formula bar but not in the cell.

Good luck!