Fallout 3, Locked out of Oasis??!!!

I travelled to oasis and was greeted by an old dude, he explained that Harold was expecting me, and I followed the quest. I talked to a man in oasis outside the entrance to the cave, and he gave me a key. Once inside the cave I had gotten so far, and couldnt carry anymore, so I exited the cave to return later, and used fast travel to dump my stuff in Springville. When I fast travelled back, oasis was locked and requires a key! I can't get back into oasis! Has this happened to anybody else???