Run the logs
Hopefully, your team uses something like World of Logs. Go ahead and fire up your favorite log system and take a look at the numbers. Start figuring out what happened.
I tend to check the DPS first. That's not because it's more likely that a bad night was the DPS's fault, it's just that it's fairly easy to look at the damage output and compare it to normal. Don't forget to check interrupts and CC breaks, as well.
If you don't get an immediate answer on what went wrong, move on to review your raid deaths. This is the next most likely place for something horrible to have gone on. In this case of raid forensics, try to look for patterns.
Obviously, look to see if the same monster or boss was responsible for many deaths. Did the same person die over and over to the same mistake? When you're doing combat log forensics, you're attempting to discern patterns amid all of the data.
Bad nights don't usually happen because of statistical aberrations. The same thing repeats. Whip out a piece of paper and write down categories of death. "Stood in fire," "pulled add aggro," and "unknown" might be three categories. Then take your best guess at what killed each person, make a mark, and tally it up.
It's important to note that log reading is not the statistical tool used by Blizzard when balancing encounters. Also, just because something appeared in the combat log and thus World of Logs doesn't mean that's the scheme of events from game. Lag plays a role, for example, and the combat log does nothing to show you where people are located.
So, grain of salt, that's all I'm saying.
If you had a one-off bad night, then I'd advise the raid leader to act like nothing's wrong. One night of wipes doesn't mean you should go swinging from the chandeliers and hurling poop about. Chances are, though, it wasn't the only bad night you ever had.
There's a concept called FUD. (It's a lot of fun to say ... fud fud fud fud.) It stands for Fear, Uncertainty, and Doubt. While the term is used in sales and politics, it actually applies very well to raiding. If you have a bad night -- or many bad nights -- then FUD will start creeping into your raid. And FUD is the enemy.
You need to get ahead of FUD, and talk to your raid members. Make sure they know that you've analyzed the situation and you're making plans to correct the problems. If you don't get in there and provide a morale boost, FUD will grow quickly in your absence. The only way to stop it is to be proactive and never give FUD the chance to grow.
It can be tempting to make every night a raid night and keep hurling yourselves against a difficult boss. After all, that's the content you have to beat, right? The problem here is that you're leaving your team in a position to fail with one another much more often than to succeed with one another.
Take some nights off to run heroics (even if you don't need them), fight in battlegrounds, or otherwise just do something that's pure fun. You need to let your team bond with one another over something that isn't wiping.
Reestablish friendships, remind your team that they like playing with one another, and get some happy times under your belt. This is especially important for those players who only show up when they need something. If Bob the Rogue is only around for raids but never anything else, your raid won't feel as emotionally connected to Bob -- normally not a big deal, but if you're fighting morale problems, it'll be too easy to blame Bob for problems.
One bad night
One bad raid night shouldn't be a big deal. If your team is struggling with progression, though, one raid night can feel like a disaster. Try to communicate before morale dwindles, and make sure you lead events to help the raid team bond with one another. Fight FUD and bad morale before they grow into larger issues.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.