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.
Greg's mobile phone is rebooting. Leo says that usually indicates a hardware issue. He can eliminate software issues by doing a hard reset, but it's more likely a memory or storage issue. If it's out of warranty, then it will be time to upgrade. He could try bringing it to his carrier store and tell them he'd re-up with them if they replace or upgrade it.