Computer not booting up at all, slow computer, apps unaccessable, missing files, computer hanging up/crashing, apps suddenly stop working/crashing, repair disk fails to repair...there are quite a few more. If you suspect it is about to die, be sure and save any important files on another drive and make sure back ups are current while it still can. Hopefully you have had TM set up and better yet CCC or Super Duper.
Last Summer I began having these issues and just as soon as I made sure everything was backed up...it died to never work again.