K4 Stuck on "Please enter the serial number"

Hi everyone

My Kindle 4 button on/off has stopped working and I've activated the "ENABLE_DIAGS" mode trying to find some option to completely disable the screensaver and... I've done it all wrong :(

After entering on the diags mode, I click on the first option (I think it's something like "Device Settings") and by accident :smack:, I entered the first option "Set Serial Number", now my device is stucked there, on a screen that says: "Please enter the serial number:", none of the buttons produce a change.

I have tried from the computer to use the fastboot to reboot it, but I always get a message "waiting for device".

Any idea how I can get out of the serial number window (without using the on/off button, of course) and try to get back to KindleOS?

Thanks!