"The setup: Your site is down. It's a small cluster configuration with only two nodes and redundant nic's, attached network disk, etc. All you know is that the problem is with VCS (although it's probably indirectly due to a hardware issue). Something has gone wrong with VCS and it's, obviously, not responding correctly to whatever terrible accident of nature has occurred. You don't have much more to go on than that. The person you receive your briefing from thinks the entire clustered server set up (hardware, software, cabling, power, etc) is a bookmark in IE ;)
"Now, one by one, in a fashion that zigs on purpose, but has a tendency to zag, here are a few things to look at right off the bat when assessing a situation like this one. Perhaps next week, we'll look into more advanced troubleshooting (and, of course, you can find lots of specific "weird VCS problem" solutions in our VCS archives)