Years ago a British Airways 747 was flying from London to Australia and while flying over Indonesia they flew through a thin cloud of volcanic ash that nobody knew was there. The ash caused all 4 engines to shut down. The crew eventually managed to get at least two or three of the engines restarted but the report I read said that at one point there were passengers kneeling in the aisles praying.
Yes, I remember that being in the news. Close call!