You could try
Ctrl-Alt-F9 to force a Full calculation
or if the dependency tree is messed up you can rebuild it using
ctrl-shift-alt-F9 - sometimes after this a normal F9 will work.
If this does not work you may have hit one of the array formulae
limits.
For excel versions prior to Excel 2007 there is a limit to the number
of array formulae that can refer to other sheets.
from KB 166342
Maximum array formulas
In Excel 2003 and in earlier versions of Excel, a single worksheet may
contain a maximum of 65,472 array formulas that refer to another
worksheet. If you want to use more formulas, split the data into
multiple worksheets so that there are fewer than 65,472 references to
a single worksheet.
For example, in Sheet1 of a workbook, you can create the following
items:
* 65,472 array formulas that refer to Sheet2
* 65,472 array formulas that refer to Sheet3
* 65,472 array formulas that refer to Sheet4
(If you are using multi-cell array formulae I suspect that the limit
is actually 65472 cells)
regards
Charles