This could be useful for others...
Seems that I have killed my MP4 USB port...
I was trying to find a rockchip firmware for a 2.2" 224x176 screen.
I load a wrong firmware: Ramos RM100 for rockchip 2606A, as it doesn't work, I plug in the USB port using the key combination to get a safe connection, upgrade connectio (rockusb)... the blank screen started to flash on every combination i do.
After that never again the device was recognized by the computer... in no way... including short circuiting of flash memory.
DEAD!!!
RAMOS firmware seems to be a lot different than others rockchip firmware!
happily was the lovely device I had for testing purposes

I never heard of that happening before, that is scary.

Daremo,
Did you use a ConsumerUpdate different to the one that came with your player?

I ask because I tried to flash a firmware (Onda I think) using another ConsumerUpdate and I killed my internal flash memory, got it back in the end but I had to flash again a couple of times.

I think part of the firmware is in the ConsumerUpdate install, could it be the bit that configures the internal memory and drives the USB port?

don't panic!!!
Some devices requires to be switched ON to allow USB connection... my Meizu M3 acts so.
Maybe this RAMOS RM100 need to be in that status to recognize USB signal... and RAMOS firmware maybe do not have a short cut (like the nice combination you have explained) to put the device in upgrade mode (rockusb)
I am dealing with it right now, any further advance will be explained here.

IT'S BACK!!!!!!!!!!!!!!!
With my eyes full of tears I have to inform the whole community that my lil device is back!... Still with blank screen but ALIVE!!!!!!!!!!

That of the USB port was a false alarm... so don't panic... was just this damn RAMOS firmware requires some tricks to get USB connected. I've read in some pages that Ramos user claims that their devices don't show itself in the USB icon.
I keep trying short circuit until I finally get the loved rockusb icon in my PC. Then I installed the firmware sent by the manufacturer (in the wrong version) and i have my pendrive back showing its blank face.
This proves 3 things:
a. Ramos devices are very strange. This is the second time I get into problem trying their firmware... first was bad blocks issue.
b. Rockchip device are extremely consistent... which means that you can load any firmware the processor will work accordingly you just will act "blind folded"
c. Sooner or later short circuit strategy will recover your rockchip device... I've done at least 50 times before i get the upgrade connection.
So here I am with my 2.2" 224x176 device ready for continue trying other firmwares that the damn manufacturer is not capable to send me.
That company is named LDX or LIDAXUN for the device LD-696.

Pleased to hear you got it back again.

This will be useful information and give hope for people who have dead players, even if the USB port is dead you can still get it back using the hardware method, but it sometimes needs a little determination.

It's hard to find a matching firmware, I have only found ONE that works and I must have tried about 20 firmwares for my screen size.

We still need that "holy grail" a Rockchip firmware extracting utility.