I have the following piece of code. It looks to see if a cell is zero

If Range("Sheet1!J" & I).Value = 0 Then

**** This is where I need assistance. If it is zero, then I need to do a
lookup on the value of
left(range("Sheet1!A" & i), 3) in another worksheet (Sheet2, column B). It
is trying to find a match on the first three digits of column A to Sheet2,
column A, and return the lookup value of B to column Sheet1!J.

range("Sheet1!j" & I).value =

*****
End If

I have been trying to get this to work for a couple of hours now, any
assistance is much appreciated.