Bounce the wheel on the stop while working the key back and forth and you may be lucky and get it to turn one more time.
This is usually preceded by reluctant key operation for a while, my usual advice it to replace the tumbler and keys the instant it gets ratchety so you don't get stuck like you are.
A NEW key from the dealer or your indy mechanic may work well enough to get the tumbler out. If not, you will have to grind the pin off so you can remove the lock assembly, which will then need to be replaced -- order it with the new tumbler and key. There is also a plastic part that usually breaks, too.
Try the new key, you can usually get one in a couple days.
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!