The issue that often causes this behavior is what’s called a kernel panic. If that’s the case, you’ll see a window like this once you get back into your user account:
Do as it says and click “Report,” and a dialog box with a bunch of technical gobbledegook will appear. You can click the triangle next to “Comments” to type info on what you were doing when the panic occurred, and choosing “OK” will send the report along to Apple.
So after you’ve told Apple’s engineers what happened, what do you actually do? Unfortunately, kernel panics are often hard to resolve, as all sorts of software and hardware components can be at fault. Here, though, are some steps to take (and some sites you can visit) to troubleshoot before you set foot in an Apple Store or call anyone for help.
The first thing to know is that one of the most common causes of these kernel panics is a misbehaving peripheral device—i.e., anything you’ve got plugged into your computer aside from your power cord. So as a first step, try unplugging everything you can and working for a while, and if the panics stop, you can then plug your peripherals back in one at a time to attempt to isolate the misbehaving one. If you’ve got a third-party wired mouse or keyboard, I’d even suggest you try a different set.
No dice? Then remember that technical gobbledegook we sent to Apple at the beginning? Well, you can view that log at any point, and searching the Web for some of its contents (or posting it to the Apple Support Communities website) may help you find out if, for example, a third-party program is causing your woes. To check it out, you’ll click on the Finder icon in the Dock, and then choose the “Go” menu from the top of your screen. From there, click the “Go to Folder” option.
When you see the little box that pops up next, type in the following and press “Go”:
Then you can scan through the contents of that folder for a log that starts with “kernel,” as I’ve done below. If your panics have happened multiple times, you’ll have more than one log entry.
Double-click the log to open it in the Console program, and then you can view it or maybe copy and paste its contents online to get suggestions. You could also look through Apple’s technical note on interpreting kernel panics to try to find some answers.
If that doesn’t work, another easy, low-risk troubleshooting step would be to reinstall OS X. Make sure your backups are current first, of course, but then follow the instructions on Apple’s support pages to do so.
Finally, if you’re just not into messing with the log files or anything like that (and you don’t want to visit the Genius Bar!), you should check out Apple’s exhaustive support article on troubleshooting kernel panics. Unfortunately, if you can’t isolate the problem to one of your peripherals or apps or fix it with a simple reinstall, you may have to take some drastic steps, up to and including wiping your drive and restoring from backup. Kernel panics are just no fun at all.
Unless you’re a writer who gets to force your machine to have them in order to take screenshots for an article! Then I gotta admit that they’re pretty fun.