Have you tried closing Excel and reopening it? Does the problem persist?

Have you changed the resolution of your monitor (or installed a new video
driver)?

Did your spreadsheet just grow past 100, 1000, or 10000 rows? Excel needs to
calculate a new width for the row number field if you did.

How long has it been since you rebooted your computer? A reboot can clear up
a multitude of problems.