[AccessD] Quick question for confirmation

There is probably actually two question here. One I want to confirm something w/y'all, and then a 2nd is just something different that I am seeing...or actually not seeing...
 
First, I wrote this program probably 13-14 yrs ago, which our Hotline people use to track "clients." Hotline is the line you call when you're looking to do harm to yourself and others, and you are smart enough to ask for help first, so its been used 24 hrs/day, 7 days/wk, for all these years. I've tweaked a couple of things and made it work w/a couple different versions over the years, but now, for the first time, they are having the issue of only one person being able to use it at once.
 
I was really just picking up Access at the time I wrote this, so I think I just didn't have the whole "Front-End/Back-End" thing down. And, since then I've just had the attitude, "if it ain't broke..." and have just left it alone. 
 
But, they have two operators and they want them both using it. I told them, "It really wasn't designed for simultaneous use by multiple people. I know it DID work, but to be honest THAT is what the anomaly was, and not that it won't work now." I've explained to them that I can, relatively easily, make this a mutli-user program, and it would be relatively quick.
 
I just wanted to pick the brains of this group first though. Basically, I want to know the same thing they do...I just don't have an answer for them...WHY did this work, as a single MDB, for all these years, and now it just stops?
 
And here is question 2...I remoted into the 2nd user's PC and I had user 1 start the program. When I try to run it, not only will it not run, but it is the first time I've ever gotten NO response...no errors...nothing...like I'm clicking on a dead icon. I tried to run it right from the directory, taking the shortcut out of the mix and nothing. As soon as user 1 exits the program, I can enter normally again. 
 
I have always gotten an error when trying to run a program somebody is already in, and never received no feedback at all. What gives here?

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