The Mac runs a series of hardware tests when you start up the machine. If these tests fail, you may see the infamous "Sad Mac" icon, hear error tones, or both.
Treatment 1: Check your RAM.
Have you just changed the RAM in your Mac or moved the Mac from one place to another? More often than not, error tones are the result of a RAM module that's not seated properly or that's incompatible with your Mac.
Treatment 2: Check other installed cards.
It's possible that another kind of add-in circuit board is causing the problem. Try reseating any AGP, PCI, PDS, or NuBus cards in your Mac.
Treatment 3: Try a different startup disk.
Although this situation is rare, error tones can occur if your startup drive is under the weather. Try booting your Mac with the System software disk that it shipped with. To start up from this CD, insert it and hold down the C key until the Mac's welcome screen appears. If the C-key trick doesn't work, try pressing Command-Shift-Option-Delete. This key combination tells the Mac to try to boot from a device other than the startup disk.
Treatment 4: Think back.
Have you changed anything since you last used your Macadded a new PCI card or peripheral, for example? If so, you've found the likely culprit. Undo your recent actions, and try booting your Mac again. If the Mac starts up properly, you'll need to troubleshoot this new acquisition.
Treatment 5: Call Apple.
As I mentioned earlier, a call to Apple may be the right solution when you're otherwise stumped.