Collapsed gas tank

2 years ago, I purchased a 1984 300D Mercedes. It really had been driven once a week by a little old lady. Was running fine, and got better the more I drove it. I would go to about 700 km, and then fill up. About one year later, on a long trip, the car odometer read 443 km, and the car stopped running. CAA came and "towed" it to a service station where we put gas (yes, I've always put diesel in it) in the car, and it started up, no problem. Since then, it runs out of gas at about 440 kms. Also, when I look in the trunk, I can see that there's a large indentation in the gasoline tank. Why did it suddenly collapse, and is there a way in which I can have it repaired?