Simple: it can't find a control called "lbltotal" in the row, so it returns a
null
value.
When you try to set the Text property of a null value it throws a "Object reference is required" exception.
So look at your row, and work out why it doesn't have "lbltotal". There are so many possible reasons why, but we can't see your code, so we can't tell you.
Start by using the debugger and looking at e.Row in detail immediately after the line:
Label lblTotal=(Label)e.Row.FindControl("lbltotal");
You need to know what it does contain, so you can look at what you thought it contained!