The drain hose is plugged. I believe it is on the top of the tranny tunnel, but I'm not sure. You will need to get the car up on jackstands or on a hoist to clear it from underneath.
Easy fix unless there is a dead mouse or a mouse nest in it plugging it up -- they won't fit down the hose very easily!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!