Usually when one turns on their Macintosh, it smiles at them (Happy Mac). But once in a long while it kind of snarls at you. First, lets explain what happens the moment you flick the switch, or hit the power key inside your Macintosh.
Once your Macintosh is brought up to full power the Mac runs what's called in the computer world a Power On Self Test (POST). Several memory and system diagnostic tests take place. If any one of these test fails, the Sad Macintosh icon appears. The Macintosh SE will have two 8-digit hexadecimal numbers displayed under the icon, remarkable similar to the icon pictured above.
How do I make a Sad Mac happy again?
Most hardware failures that display a Sad Macintosh error will do so before the floppy drive or hard drive start spinning. If a Sad Macintosh appears after the disk starts spinning, the first digits of the error code are usually 'OF' and often indicate corrupted software. If you get this error code, try restarting the Macintosh with the Option and Command keys held down to rebuild the desktop file. You also may be able to fix these problems by reinstalling system software.
If all else fails it means their is a problem with the hardware and you should take your Mac to an Apple Qualified technician. Attempting repairs yourself will void Apple's warranty, of course, you don't have to tell them you tried to fix it yourself...
If you've never seen a Sad Mac before and you have this urge, try hitting the interupt button (plastic things on the side of compact macs, little buttons on the front of newer Macs, or some weird keyboard combination on others) while the Mac is checking the RAM (this is when the screen is grey and and void of all else at start up). Of course, this is not recommended by anybody because I'm sure there is some way it might affect something adversaly.
What do those numbers mean anyways?
Macintosh SE and later systems:
Error codes 0001 through 000E indicate hardware failures.
xxxx0001 ROM test failed
xxxxxxxx
xxxx0002 RAM test failed
xxxxxxxx
xxxx0003 RAM test failed
xxxxxxxx
xxxx0004 RAM test failed
xxxxxxxx
xxxx0005 RAM test failed
xxxxxxxx
xxxx0006 VIA1 chip failed
xxxxxxxx
xxxx0007 VIA2 chip failed
xxxxxxxx
xxxx0008 ADB failed
xxxxxxxx
xxxx0009 MMU failed
xxxxxxxx
xxxx000A NuBus failed
xxxxxxxx
xxxx000B SCSI chip failed
xxxxxxxx
xxxx000C IWM chip failed
xxxxxxxx
xxxx000D SCC chip failed
xxxxxxxx
xxxx000E Data bus test failed
xxxxxxxx
Error code 000F is typically associated with software problems or hardware that has been modified
xxxx000F Bus error
00000001
xxxx000F Address error
00000002
xxxx000F Illegal instruction error
00000003
xxxx000F Divide-by-zero error
00000004
xxxx000F Check instruction error
00000005
xxxx000F coTrapCC, TrapCC, or TrapV error
00000006
xxxx000F Privilege violation
00000007
xxxx000F Trace
00000008
xxxx000F Line A error
00000009
xxxx000F Line F error
0000000A
xxxx000F Unassigned error
0000000B
xxxx000F Coprocessor protocol violation
0000000C
xxxx000F Format exception
0000000D
xxxx000F Spurious interrupt
0000000E
xxxx000F Trap 0 to 15 exception
0000000F
xxxx000F Interrupt level 1
00000010
xxxx000F Interrupt level 2
00000011
xxxx000F Interrupt level 3
00000012
xxxx000F Interrupt level 4
00000013
xxxx000F Interrupt level 5
00000014
xxxx000F Interrupt level 6
00000015
xxxx000F Interrupt level 7
00000016
xxxx000F Coprocessor BRA or SET on unordered condition