If I understand correctly you are asking if you can have Nagios automatically restart a service when it detects it is down? Yes, this is called an Event Handler. It is basically a script (perl, VB, batch file, etc.) that you create and tell Nagios to execute in response to a specific event.
This is the documentation page regarding setting up event handlers, I think it would be the best place to start.
nagios.sourceforge.net/docs/3_0/ ... dlers.html
A warning though, Event handlers aren't a substitute for notifications or good administration, someone should always check when a service goes down. The reason for this is because if the service goes down for some other reason the handler can restart it but that won't fix the problem, so it will just crash again. Sometimes restarting over and over can actually make things worse, so use care when setting them up.