When there is an outage, it is natural to panic a little and maybe try a rush troubleshooting steps.  In my experience this is oftentimes counterproductive.  Symptoms can often be missed as the root cause, which has the net effect of extending the time to resolution.  So where would I suggest is the best place to start, well that in part depends on the symptoms.  However, the log files will tell you everything that you need to know.