The macro should only leave Excel open if it was open before running the process.
The subscript out of range error suggests that VBA has crashed irrecoverably (in which case you need to reboot) or the criteria set at the top of the macro to match your data i.e.
Code:
Const strWorkBook As String = "Count0To12Hr.xls" 'The name of the attached workbook
Const strSheet As String = "Count0To12Hr" 'The name of the worksheet to process
Const strCell As String = "E2" 'The cell to process
Const iCount As Integer = 19 'The threshold value of the above cell
Const strMessage As String = "This is the reply message body text." 'The default signature will be included.
What is the range of values that could appear in E2?
What is the range of values that you might put in iCount?
Look up the numeric ranges for Integer and Long variables.
It might be better to change
Const iCount As
Integer = 19
to
Const iCount As
Long = 19