land based radar only extands so far It probably went off radar on schedule but then failed to check in at the next point which could be because it had already crashed. As for the reason well it was in the middle of a storm that is building into a hurricane.
I'm going for rapid decompression Which knocked out amongst other things, the planes electrical system which caused the alarm to go off. Or as a back up guess i'll go for a lightning strike causing an electrical fault (thus the alarm) which took the co-pilots attention away from the weather he was flying into and eventually caused the plane to crash. I say co-pilot because the pilot was probably not at the controls at the time.
JJ Abrams has sent it back in time to a desert island. A bit like that Oceanic flight a few years ago. Flight 815, wasn't it??? Think there were a few survivors too. Matthew Fox springs to mind.