Try
ThisWorkbook.Sheets("name of sheet 2").Range("A1")
to access a range in sheet 2 independently of where your code is or which sheet is currently active. To make sheet 2 the active sheet, try
ThisWorkbook.Sheets("name of sheet 2").Activate
If you just need the sum of a row in a different sheet, there is no need for using VBA at all. Enter a formula like this in sheet 1:
=SUM([Name-Of-Sheet2]!A1:D1)