segfault when running Minecraft-downloaded java runtime executable

I have purchased Minecraft and after a year or so it has stopped working.

I have tried to troubleshoot things a bit and found that the java runtime which is downloaded by the launcher does not work : it goes right into a segfault when launched from the command line. On the launcher the result is (translated from french) : "an unexpected error has occurred and the game has crashed. We are sorry for the disturbance caused" and "return code : 0".

Well, you guys are not remotely as sorry as I am, having to explain my boy that he can't play his favourite game any more.