Turning Engagement Program back on

I was wondering exactly what happens when you turn an engagement program back on.  We had a campaign running where the leads received the 1st email, but then they wanted to turn the program off due to some issues that had to be addressed before the 2nd email went out.  Now we are ready to start the program again, so if we turn the program back on, will the leads resume where they were in the stream, or will they receive Email 1 again and start over?  Thanks!