why formula does not work in excel?

In our LAN, one colleague asks for my help:
He has an excel file, with some formula defined. But now the result doesn?t change after he changes the source value (in same sheet).
I then tried, first open a new blank file, formula works properly. But after opening that file, immediately formula stop working for changes, even opening a new file.
I suspect those files contain virus, but the Anti-Virus software cannot find any virus from that computer.
Furthermore, I tried to open that file in another computer, and the formula doesn’t work either.
It seems the formula cannot refresh data.
What might be the reason?