More than just irksome, these symptoms mean serious business…
6. Mac won't start
Installed Snow Leopard and got a grey screen? Check out http://support.apple.com/kb/TS2990, as it could be due to an incomplete previous update.
Plugged in a USB or FireWire device? It could be incompatible or conflict with another peripheral; unplug everything and restart. Memory upgrades can sometimes get through the POST (see Tip 6) but then crash; try re-installing. The same goes for other internal upgrades.
Nothing so obvious? Try a Safe Boot, which adds extra self tests. Shut down, then start up while holding Shift. If this works, immediately try a normal restart.
Otherwise, reset the PRAM. To do this hold down Option+Cmd+P+R before the grey screen appears; check the hard disk (Tip 15); and read this page and this page.
9. Kernel panic
When the OS crashes it can only bow out with the message 'You need to restart your computer'. This is a kernel panic. It's rare and, thankfully, often fixed by restarting.
If not, try unplugging all add-ons from your Mac and restarting. Internal upgrades – including dodgy RAM – can provoke kernel panics, as can devices such as USB hubs. Another possibility is that a system file has become corrupted.
Restart while holding x; if this works, go to System Preferences, click Startup Disk and ensure the correct disk is selected. Or, restart holding down Option, releasing it when icons appear; you'll be asked which disk to start up from. Or zap the PRAM (see the tip above).
If none of those work, perhaps you've moved, renamed or deleted a system file. Reversing this may sort it out – or not.
The last resort is to reinstall Mac OS X. Start up from your Mac's OS X DVD, run the installer and choose Archive and Install. You'll need to reinstall your apps, though.
10. An application is playing up
When an application suddenly no longer works properly, a common cause is corrupted preferences. Each application has its own preference files containing configuration data, and because they're read and written so often these are rather susceptible to corruption.
When an app crashes, Mac OS X may offer to reset its preferences. You can also do this manually.
Preference files, identified by the extension .plist, are stored in /Library/Preferences (on your main hard disk) and in /user/Library/Preferences, where user is your user account's home folder. Look here and you can usually see which prefs belong to which apps.
Having quit the application, make a folder on the Desktop called Suspect prefs (for example) and move all the app's preference files into this. Now relaunch the app. This forces it to rebuild its plists, which should solve the problem.
If it doesn't, just put the plists back. Some apps also have self-repair functions; for example, holding the Option key while launching may bring up a settings box. Have a look in your app's manual. Otherwise, you'll have to reinstall the application from its DVD or disc image file (.dmg).