Optiplex won't start

I have an old dell opti plex 745.  Several months ago I started having problems, It has been so long since I've been able to start the machine, I can't remember what was happening.  I shut down the computer because of the problem what ever it was, and now it will not restart.  I push the power button and nothing happens.  The monitor has power so the computer should as well.  Any Idea what might have happened?  Thanks