I've had this problem now that seems to have infected my computer for several days. Every Excel file that I open will not automatically update formula cells. I have to click in the formula bar and hit enter to make the cell update.

Someone mentioned having another Excel file open but I would never set calculation to manual and have only been dealing with one file at a time. Got me thinking... I hit Control+alt+delete and went into task manager and sorted all of my running processes by name and sure enough, there was a rogue Excel.exe process running. I killed it.

Reopened Excel and all of my numbers instantly recalculated for me on the open worksheet.

FTW