It could mean a few things.
1. The columnwidth is too narrow to show the number.
Widen the column or change the font size of that cell. Or change the
numberformat to General.
2. You have a date/time in that cell and it's negative
Don't use negative dates. If excel was helping you, it may have
changed the format to a date. Change it back to General (or some
other number format).
If you need to see negative date/times:
Tools|options|Calculation Tab|and check 1904 date system
(but this can cause trouble--watch what happens to your dates
and watch what happens when you copy|paste dates to a different
workbook that doesn't use this setting)
3. You have a lot of text in the cell, the cell is formatted as Text.
Format the cell as general.
4. You really have ###'s in that cell.
Clean up that cell.
5. You have # in a cell, but it's format is set to Fill.
Change the format
(format|cells|alignment tab|horizontal box, change it to General.
