Ahh, the Doom of Valyria. Always an interesting topic. Here is what we know:
The Doom is said to have descended on Valyria and destroyed it in a single night. However, it is also said that the Doom continues to hold Valyria in its grasp. So the Doom itself took place on a single night but it's after-effects continue to linger. The City of Valyria is uninhabited and in ruins, but not completely destroyed according to AGoT, ASoS and AFFC. We are also told that the Smoking Sea between Valyria and the mainland is 'demon-haunted'. The Doom is also said to have ended the existence of magic in the western world and destroyed the Valyrian dragons. According to an old (abandoned) pre-release blurb for A Feast for Crows, the Doom 'shattered the Valyrian Peninsula'. The Faceless Men also apparently started life working in the Valyrian slave-mines under the Fourteen Fires (where dragons laired) and vowed vengeance upon Valyria.
Conclusion:
The Doom of Valyria was a colossal super-volcanic eruption when fourteen massive volcanos stretching across the neck of the Valyrian Peninsula exploded. The Valyrians somehow built mines under these volcanos without killing the slave-workers (although the conditions were not pleasent), suggesting magic was used. The theory is that the vengeful Faceless Men somehow deactivated or switched off the magic protecting the mines, causing the volcanos to erupt one after the other. Such an explosion would probably exceed anything seen on Earth in recent millennia: imagine an eruption ten times the power of Vesuvius or Krakatoa. The Valyrian Peninsula was shattered, the burnt-out remnants of the volcanos tumbling into the newly-formed Smoking Sea. Such a calamity could easily lead to the seabed in the area remaining volcanically active even 400 years later, leading to boiling waters and steam drifting acrosss the surface (the 'demons' reported by sailors; poisonous volcanic fumes would kill sailors, strengthening the reports of evil magic at work in the region).
Valyria is far enough away from the volcanos (300 miles+, assuming the ASoS Slaver's Bay map is to the same scale as the Westeros ones) not to have been destroyed by the initial explosion, but close enough to have been laid waste by the dust and volcanic debris tumbling out of the atmosphere. Like with Pompeii, there was probably enough time for some people to escape the Doom (explaining the paintings of it that exist from first-hand sources).
Naturally, if the Valyrian dragons lair on the Fourteen Fires, they would have been slain in the volcanic eruption, explaining the deaths of the Valyrian dragons save those few (or possibly their eggs) in safekeeping on distant Dragonstone, which had been a Targaryen holding for a century by that point. This would also have removed magic from the western world (although, again, some magic would have returned once the Targaryen eggs had hatched).
Maybe they killed themselves with their own power.
