No display, with fan running at top speed

You tried the disconnecting the power cable and holding down the power button for 30 seconds.

Have you tried this with also taking out/disconnecting the battery (check in manual how to do this)?

Otherwise it could be a motherboard, CMOS battery or a RAM problem.

Replace CMOS battery video

I see in your case you removed the CMOS coin battery but did you check its voltage (idea around 3 VDC) or replace it?

Some other possiblities:

HP Laptop BIOS Recovery Win and B or V Key

• Check Key Combination: First, make sure you are pressing the correct key combination. On some HP laptops, the key combination for BIOS recovery might be different. It's usually "Windows + B" or "Windows + V," but it could vary. Check your laptop's manual or HP's official support documentation for the correct key combination.
• Power Cycle: Turn off your laptop completely and unplug it from any power source. Then, press and hold the key combination ("Windows + B" or "Windows + V") and while holding it down, press the power button. Keep holding the key combination until the laptop's screen lights up or you see any signs of BIOS recovery.
• Use a USB Drive: If the key combination method isn't working, you might need to use a USB drive with a specific BIOS recovery file. Visit HP's official support website, enter your laptop's model number, and look for the BIOS recovery files and instructions for your specific model.

Let us know how it goes.