mike wrote:
The battery has gone from not working to running the laptop, so looks
like it's actually charging.

Have you tried running it on just the AC adapter, with the battery
removed? Is the AC adapter the original one that goes with this
machine, or a good one that may not match the machine, or a random
Chinese "120 W Laptop Power Supply Top Best only $1.37" thing?

Press the power button.
Fans run
CD spins up
No light on the screen.
Nothing on the VGA external monitor

Did you try an HDMI monitor? It might be set to "prefer" that when
starting.

Have you tried shining a strong flashlight at the built-in monitor when
booting? Sometimes if the backlight is broken, you can at least see if
it's trying to put any text on the screen that way.

Does it beep? How about plugging some powered speakers into the audio
jack at bootup? Startup chimes (rather than beeps) are more of an Apple
thing, but maybe it's trying to tell your ears something.

Is there a built-in SD/microSD card reader that might have a card in it?
If so, eject the card (if you can), and try again.

If I remove the RAM, it powers up and the fans run
continuously.
Replacing the RAM doesn't help.

Do you know if *all* the fans are working? Sometimes laptops refuse to
run if the (internal) main CPU fan isn't turning, or isn't turning fast
enough. Maybe put your fingers or some tissue paper near the vent slots
while booting, to see if any air is moving.

There are a LOT of variables on the bios file formats and file names
and keypresses required to initiate the USB recovery process.

I've never tried to update a UEFI BIOS, either under normal or rescue
conditions, so I don't have any specific advice on that.

Matt Roberds