Help With Error: Subscript out of range

Hi,
I'm no expert with VB, I've just done the odd simple program from time to time.

I have a program that's been given to me by its author, which he created some time ago, that has developed a problem.
It used to function OK, but now when the application is run:

The program starts it process, but then stops with this popup error message:
Error : 9
Description : Subscript out of range
LineNumber : 7250

I get the same message when I RUN the program via the VB Project Window.
This popup is not the standard grey VB popup error message,
but a bespoke error-handler, so when the error occurs, the Project Window doesn't hi-lite the part of the code that is causing the problem.

So my initial queries are:
Q1: In a previously working app, what kinds of problems can cause the above error?
Q2: How can I get the suspect code to be hi-lited in the the Project Window?
Q3: Any suggestions how to proceed with this type of problem.

Many thanks.