Xuanlong ault rifle disappeared. Help?

Hey, I got Fallout 3 GOTY edition. I played through it and got the rifle without even knowing about it. Thought it was an awesome gun (Best gun in my entire collection except for big guns) anyways, I was on the mission to retrieve the G.E.C.K. and used it to attack the guys Fawkes was attacking, but after I killed one of them, I still had my gun out and it was all fine and dandy. Then I switched weapons to my regular ault rifle so I wouldn't wear out the Xuanlong, but when we were attacked by two more super mutant brutes and I went to take it, but it didnt appear in my hands. Sometimes I hit the wrong directional pad key so I ignored it until after the battle. I then went into my inventory, it was there, I repaired it with chinese ault rifle pieces and then went out. I then hit the right hotkey but it didnt appear. I went in my inventory. It wasn't there either. It was just GONE! So, if anyone has any ideas or suggestions on what happened or how I can fix it would be much appreciated. Thanks.