The process should be:
* power on the box
* manually restart the datanode and tasktracker services (these will not start automatically when you start the computer)
* once these services are started they should join the cluster automatically, provided that the namenode is already running.
This is assuming that you have fixed whatever it was that caused the datanode to die in the first place. Also you should note that the proper procedure for shutting down a node, if you cannot stop the whole cluster, is to stop the hadoop services on the box first then after they have fully stopped shutdown the computer.