Format that offending cell as General (Format|Cells|Number tab)
and try reentering your formula.

Michael wrote:

When I enter a function, for example '=UPPER(A1)' where A1= john, the
following is displayed '=UPPER(A1)', not 'JOHN'. If I use the function wizard
the answer is displayed correctly as well

Other functions within the worksheet are working normally. For example ,
=DATEDIF(H17,I17,"y") returns a number not the function.

Any suggestions gratefully received.

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Dave Peterson