Hi,
Couple of things to check.
Do you have values for all the points on your line, that is no gaps in
your data. If you have empty cells select the chart and then check, via
the menu Tools > Options, the chart tab settings for plotting empty cells.
Do you have a line style selected, as it is possible to plot points
without a joining line. Double click a point and check the Patterns tab
for formats being applied to the line.
Cheers
Andy