A list of the most likely causes really depends a lot on what model/year car you have. For instance, the fuel pump relay Rob suggests applies to all but 83 and 84 non turbos. The 25A fuses Eric suggests are used between 83 and 90 thereabouts. Timing belt is universal quick stall. Crank angle sensor is for 89 and up. Alternator brushes can make a car die kinda dead, to where your dome light and clock is all you have left. Make sure you roll the fuses around in the fuse panel to make sure they are making good contact.

How dead is she?
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Art Benstein near Baltimore

Check the famous 25amp fuse that's between the battery and strut tower on the driver's side in the engine bay. I was in a parking lot going to park when my car stalled on me. It turned out to be a corroded fuse holder and fuse. My fuse holder was black and burnt, it actually snapped in half!
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Regards,
Eric Staufer,
'89 244DL 118k
My 240 Page

Well, more detail would be good, but once you verify that the timing belt didn't break, you should replace your fuel pump relay, and if it's a 1989 or newer, replace the engine speed sensor (crank sensor) as well.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total

"...replace your fuel pump relay..."

Resolder, resolder, resolder.

But only once.

$0.

Even buying a soldering iron 'n supplies at Radio Shack is cheaper than buying a relay.
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Don Foster (near Cape Cod, MA)

Yeah I know and I do resolder mine.... however when getting a car going on th side of the road, you usually need a second one to just plug in. Then you can re-solder the old one, and keep it forever as a spare.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total

"Then you can re-solder the old one, and keep it forever as a spare. "

The way those things are soldered at the factory, you'd be running with the superior part in your spares box.

...'not that there's anything wrong with that'...
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Art Benstein near Baltimore

That's a good plan.

Fortunately, wife's relay failed close to home. So I resoldered every relay in the car.

The one time I was caught with my pants down ('82 B21), I simply jumped fuse 4 to 6 (or is it 5 to 7?) with a paper clip and drove 25 miles home.

Then I resoldered 'em all.

Having a spare is a great idea. Having it means you'll NEVER need it.
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Don Foster (near Cape Cod, MA)

Yeah the fuse trick works great. My buddy's 80 244 fp relay failed so we just used the jumper for the week he couldn't work on the car. I think it is 5 and 7. You'll know right away as the fuel pump will start whirring without the key in. The only dangerous part is in an accident, the fuel pump will keep on going... but as long as you wear your seatbelt you'll be okay to pull the jumper anyhow.

"...you'll know right away as the fuel pump will start whirring without the key in. The only dangerous part is in an accident, the fuel pump will keep on going..."

You're preaching to the choir, son.

If it's 5 & 7 on the early 240s, then it's 4 & 6 on the later 240s.

I knew it was one on one and the other on the other -- I simply couldn't remember which was which, 'cause it's been a few years since I last used the trick.
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Don Foster (near Cape Cod, MA)