I have a simple spreadsheet in which three columns are used to keep track of
an account balance.

Column A is withdrawals, B is deposits, C is balance. A typical entry in C25
would be =C24 - A25 + B25. One of the cells in A or B will be empty in row
25.

This approach has been working well and blank cells have resulted in a value
of zero being used in the calculations. However, recently the #VALUE result
comes up instead of the expected result..

If I replace the empty cell with 0 then the formula appears to work
correctly, so the problem is related to how the lack of an entry in one of
the cells is being considered. The confusing thing is that entering a value
of zero has not been required previously.

All three columns have been formatted as number.