Claire,

PEH is correct. The tank changed shape permanently. It will not return to its original, or some intermediate shape without some force applied to bend the metal back. It is feasible that the tank, which starts life in a pretty structurally unstable state, is still unstable and can be convinced to return to something near its original state with a little extra air pressure on the inside. I would be a little nervous about putting much pressure in there for fear of blowing some fuel line connections off and getting Diesel fuel all over the place.

If the tank is stuck in its new shape, you might take the car to a shop to have it reworked by someone with some additional tools. Seems to me a paintless dent removal tool or something similar should work. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)