Computer won't boot after memory upgrade (old RAM doesn't work now)

Solution 1:

Lower CAS latency should not be a problem (on the contrary, new RAM is slightly faster). Crucial says this model can take 16 gigs, 8 per slot.

By what you describe, the computer probably works. This is a shot in the dark :) but it could be using the default display on boot, since you reset the BIOS to defaults. Some PCs boot with working internal and external video, but others you can choose... yes, in the BIOS...

I think this bit in Getting Started confirms it:

If the system has multiple video sources (embedded, PCI, or PCI-Express adapters) installed (embedded video on some models only) and a single monitor, the monitor must be connected to the monitor port on the source that is set as the primary video source. During boot, the other video ports are disabled; if the monitor is connected to one of these ports, the monitor will not function. You can select or change the primary video source in Computer Setup

Edit: on second thought, this is talking about a second video card, which you probably don't have... still, you could try blind BIOS for a bit before you try the disassembly option:

Try:

• Do you have a video output button like in laptops, or more than one video output to try? (I'm guessing no, but couldn't hurt to ask xD).
• Confirm computer is working:
  • Hard drive blinking with activity or making noise?
  • Can you restart the computer blindly from the BIOS? (press F10 repeatedly for five seconds after powering on, wait another five seconds, press F10 again, then Enter to save and restart)
• Find screenshots/video/a PC with a similar BIOS so you can navigate it blindly to switch primary video. From the Maintenance & Service Guide:
  • Computer Setup—Advanced > VGA Configuration: Displayed only if there is an add-in video card in the system. Allows you to specify which VGA controller will be the “boot” or primary VGA controller.
• Disassemble PC and unplug screen to force external video (probably hard work, knowing HP and AiOs... example with an HP laptop)

Edit: The BIOS in this post looks similar enough:

So you could try this:

• Press F10 repeatedly for five seconds after powering on, wait another five seconds.
• Press right arrow four times (this assumes File is selected by default; it could be five times... search for a video with a similar BIOS to know for sure )
• Press down five times (again, assuming first option is preselected...)
• Press up (try down next time if this doesn't work)
• Press F5 to set as primary, F10 to accept, F10 and Enter to save and restart
• Fingers crossed!

BIOS info screen:

P.S: I'm not 65 yo, but I wouldn't remember if my computer bleeps either. ;)