I was ready to give up and buy a new motherboard for this issue, but I managed to figure it out and wanted to put it somewhere so it could be searchable by someone troubled by something similar. I've used many a tom's hardware discussion in the past to troubleshoot lots of pc-building issues.
The m9500y has a MSI MS-7548 motherboard, which does not seem to have any sort of system speaker and made this difficult.
The HP Elite Pavilion m9500y I had randomly shut off and would no longer show anything on the screen upon boot. The power LED had turned off, as well as the other lights on the front of the computer. I managed to reset the BIOS using the notes found here (http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph...): replicated here:
Reset BIOS Version 6 or less
1. Turn on the computer and repeatedly press one of the following keyboard keys. Press the key about once every second for 5 seconds after turning the computer on.
2. For HP and Compaq computers that came with Windows 7 or Windows Vista, press F10 .
3. For HP and Compaq computers that came with Windows XP, press F1 .
4. For Compaq Presario desktop computers built prior to 2002, press F10 .
5. Wait another 5 seconds.
6. Press the F5 key. This restores the default settings of the BIOS.
7. Press the Enter key. This accepts the changes.
8. Press the F10 key. This saves the changes.
9. Press the Enter key. This saves the changes and exits from the BIOS.
10. The monitor should display the first screens received from the computer (the logo screens). If not, find and download the latest BIOS update if one is available for your computer.
11. If a BIOS update was not available or the monitor is still blank, continue to the next step.
Reset BIOS Version 7 or greater
1. Turn on the computer and repeatedly press the F10 key for eight seconds.
2. Press the Down arrow three times.
3. Press the Enter key two times.
4. Restart the computer.
Mine turned out to be the version 6 or less instructions. Then when I pressed the power button to turn it off it would start right back up again--still nothing shown on the screen. I finally unplugged the machine and rebooted, and there was the HP splash screen. It seems like it had gone into some sort of bad system resume, and the first time "Resuming Windows" did not eventually start it up. I had to choose the other option, to do a fresh restart, and then it finally booted up Windows.