My version of Excel recently started acting strange. I have a complex
spreadsheet with many cells that depend on other cells for their content.
Today, for some reason, the cells do not update themselves. If I change a
cell, the cells that use formulas to get part of their content from the
changed cell do not update automatically. I have to manually go in, click on
the formula, and hit enter in order to get them to update. I've tried
searching for information on how to get the cells to update to no avail, and
restarting my computer did nothing. How do I fix this?

Note: This is in Excel 2007.