Skip Code Depending On Cell Condition

I m trying to bypass some code if a certain condtion is meet. If Cell A1 has a zero in it I want to skip the following code and continue one with the rest of the macro. This is the code I have.

j = 0
RowCount = Cells(Cells.Rows.Count, "a").End(xlUp).Row
cell_a = Range("A1").Value
If Not cell_a = 0 Then
Rows("1:" & cell_a * 2).Select
Selection.Insert Shift:=xlDown
Range("A1").Select
For i = 1 To cell_a * 2
Range("a" & i).Select
ActiveCell.Value = j
j = j + 0.5
Next i

I put the If Not/Then statement in there to try and figure out what do. But I don't know what to put after the THEN statement. Basically I want run the code after the THEN statement if there isn't a zero in cell A1 and skip the code if A1 contains a zero.