When the muscle become deactivated it loosens up and lets the organs hang loose giving you that slight bump in your stomach. Sitting down for long periods of time deactivates the muscle!
So how do we reactivate it? Well simple, all you need to do is practice retracting your stomach inwards (sucking your belly button in and up) and holding for a short period. This can be done while sitting, walking, driving there are so many ways. Start by doing it for 10-30 seconds, 6 times a day.
This way you will start to activate you muscle and start pulling your stomach back in.