I understand how to fix the issue (switch calculating option from manual to automatic) but I am curious as to WHY it went to manual. Could it be some kind of update? I have been using these spreadsheets for a long time, and suddenly today this issue came to be. I'm using Microsoft office 2007.
Its only certain files, and they are stored in a network drive. We have not installed any updates recently to our computers, and only a handful of people are experiencing these issues.
Since these files are shared on a network drive, it is highly likely someone else who is using the files is saving them with auto-calculation turned off. This is evident because you say its only certain files. This indicates those are the files that person has saved in that fashion.
To verify this, open one of the problem files, switch it back to auto-calculation and save it again. Open it again. If the changes stay, then you need to find out who is saving them that way.
In my case, we have files over the network drive. The calculation turned to "Manual" only in case where there was a "Circular reference" by mistake in my file. That's the only reason the calculation went to manual.
