Bob put his iMac to sleep and then when he went to wake it up, he got a "bong" with a blank screen. Leo says that those tones indicates different things depending on the tone. Here's a list. It's possible the backup battery in the computer is dead and needs to be replaced. But it could also be a failed hard drive. It could also be an EFI ROM update is in progress. If you hold down the D key when booting up, you'll go into diagnostic mode. If that doesn't work, then you have a major hardware failure.
Ryan has an HP computer and it came up with a black screen. He tried to swap out the power supply, video card, removed all the USB peripherals, replaced the CMOS battery, and still has a black screen. He can hear the power supply running, and the fans are spinning up. Leo suspects it may be just a bad motherboard. Since Ryan has done all the easy stuff first, and then tried replacing peripherals, that only leads to the main motherboard. He should also listen for POST codes to see if the machine is letting him know something is wrong.
Devon thought his video card was failing so he removed it and now he's getting no post codes. Could the motherboard be bad? Leo says it could be a number of things, including loose cables or CPUs, memory etc. He should try reseating everything. Leo says Devon may have also lost the CMOS battery in the process, but that wouldn't fail the post.