I have accidentally stopped the central administration service on a share point server and can't find a way to re-activate it? Is there any way to do this from one of the SP command line utilities?
That would depend on how it was stopped. If it is simply the web site itself that has stopped, then you can open Administrative Tools -> IIS Manager, browse to the Sites -> Central Admin Web site, right click and pick Start. If it doesn't start this way, look in your event logs and see if there is anything there, like a permissions or security error.
If it is stopped some other way or if you accidentally deleted the site then that will take additional steps to recover.