I have an Excel worksheet with a strange behavior:
I click in a cell that has a Formula (one cell that was normally showing its expected result for its formula). When i leave the cell, it does not shows the value anymore - instead, it shows the Formula!
Generally the cause for this particular behavior is that your cell is formatted as Text.
To fix this error, just select the cell, set its formatting to “General”.
When a cell is formatted as Text, Excel makes no attempt to interpret the contents as a formula.
I had this exact same problem some time ago, and i found the answer here:
If the problem continue, you can find some other reasons there.