I don't know what I'm doing wrong. All I'm trying to do is add a column of
numbers. In cell L13, my formula is =SUM(L2:L12). All I get back is 0. I've
checked the formatting of the cells and they are all numeric. I have also
tried adding cells manually, e.g., =L2+L3 -- and I get the correct result.
I'm pretty sure I've used this function before without any problems. What am
I doing wrong?

I was having a little problem with setting the formatting. Most of my
spreadsheet contains wrappable text, and I had set the style for that. Then
when I tried to adjust my numeric columns, it kept applying the numeric
format to the entire worksheet until I figured out how to specify a separate
style for those columns. Could I have messed something up there?