I'm not an expert on this, but I don't think that once the sun has converted all its hydrogen to helium its the end. Once this has happened, I think stars generally then start fusing this helium together to make yet heavier elements for a few more million years. If I am correct, the energy output of the sun will decrease but not be completely reduced. Even so, this decrease in energy may be enough to cause the extinction of many species on earth. If the sun was completely extinguished, theres no doubt about it that humans and all life on earth will become extinct fairly soon.
