The best-known incident of an airliner imperiled by volcanic ash occurred in 1982 on a British Airways flight from London to Aukland. The plane, a 747, flew into an ash cloud spewed up by a volcano in Indonesia and lost all four engines.
Famously, the 747 captain, Eric Moody, in a masterpiece of British understatement, got on the intercom after the engines flamed out and said: "Ladies and gentlemen, we have a small problem. All four engines have stopped. We are doing our damnedest to get them under control. I trust you are not in too much distress."
Moody and his two flight officers (747s carried an engineer back then) managed to glide the plane in a descent from 37,000 feet to 12,000 feet, for about 15 minutes, before they got an engine restarted. The plane landed safely in Jakarta. The engines were severely damaged.
Here's an interview with the now-retired Moody yesterday by the BBC. It's well worth listening to.
It was nighttime when the 747 entered the ash cloud, and the pilots were unaware of the ash problem. Volcanic ash is dry and is not typically picked up on radar.
There was another volcanic-ash incident involving a 747 in 1989, when a KLM flight from Amsterdam to Anchorage flew into an ash cloud from the erupting Mount Redoubt. It lost all four engines and landed safely, though with severe damage to the engines and fuselage.