Mowerman

If you have no other customisations, right click on any toolbar, select
customize and then select standard from the list of toolbars and click
reset...

Or are you talking about the autosum on the status bar
at the bottom of the screen. If that is the case then
right click anywhere on the status bar and make sure
sum is ticked. You will see that there are other options
there as well.

HTH
Martin