Pull back the liner in the trunk and find the manul override lever or door lock type button and unlock it by hand. After you get it open, check the locking tang -- on the W124 it's an extension of the door with a hole in it, often gets bent over so that the lock pin hangs in it.

Straighten it out if it's bent. May need to repaint it, too, as the paint will usually crack off when it gets bent over at a 90 degree angle...
Peter

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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!