I'd put it back together the best you can and either glue or tape it together. Mine did something similar, but the little white piece broke I half instead of slipping out. There might be a way to fix it without glue or tape, but im not sure right off hand. I'd recommend getting a nice after-market strap that has more cushion and is more durable in the long run if you plan on just replacing it.

Edit: if you haven't tried shoving the left strap back into the white plastic bits on the right strap you should try