If the Tracker hangs or kills itself off for some reason (the most common reason is a badly behaved Tracker add-on), you’ll probably want to make sure it’s all the way dead before restarting it.
To kill the Tracker all the way dead, hit Ctrl+Alt+Del to bring up the Team Monitor, then kill off the Tracker’s remaining threads. To restart the Tracker, bring up the Team Monitor again and you’ll see a new button labeled „Restart Desktop“. Clicking it re-launches the Tracker. If you prefer to do these things manually, open a Terminal and type:

/system/Tracker &

similarly, the Deskbar can be restarted by typing

/system/Deskbar &

Remember that you can manually kill the Tracker with the Vulcan Death Grip. The Deskbar must be killed manually, via the command line, Team Monitor, or with a 3rd party process controller / thread manager.