It hangs as your recordset is not being incremented, you need a MoveNext
Do While Not rst.EOF
If IsNull(rst.Fields("column1").Value) Then
MsgBox "Has nulls"
End If
rst.MoveNext
Loop
Simoco had a better suggestion for accomplishing this,
If DCount(1, "table_1", "IsNull([column1])")>0 Then
MsgBox "Has nulls"
End If