How it happened:
I was playing in a campaign and during a fire fight. When one of my unit, a Rifleman, was injured. So I commanded the medic to heal the Rifleman. While the medic was doing so. He died and the unit was sort of waiting for the medic to heal him. I also tried reproducing it and wasn't able to. So I guess its a bug sometimes happens.
How I fixed him:
So I tried healing him, but it didn't work. So I got my pistol out and shot his leg to damage him. Then let my other unit, an AT Soldier, to treat him and it work. The Rifleman was now able to move and follow me orders.
Edit: Why did I shoot him? Because his health wasn't low enough for a none medic to heal him. I was trying to maximize his health earlier until the medic died trying.