Problema solucionado, gracias a todos los que me proporcionasteis ayuda

Dejo aquí la solución que encontré en el foro
http://forum.xda-developers.com por si queréis visitarlo y para quien sufra el mismo problema (en inglés):
by @
Vijay2:
first you switch off your.....try two or three times....or try again and again ....if you switch off your fone....then try to switch on in to fastboot....i have downloaded a fast boot rom..........1.16 version........extract the downloaded folder then first you click on ....z9 mini app in this folder.....and connect your mobile to pc......its start in automaticly.......wait for restart.......ohla....your mobile runs....
Archivo :
https://drive.google.com/file/d/0B14...1rME1Fd0U/view
by @
MAJklB:
This helped:
- Use a fast microSD card and copy on the root folder the original Nubia ROM (you can download it on the first page of this thread)
- Insert this microSD card into the phone
- Turn on your phone and let the battery completely discharge
- When the phone is turned off, press and hold Volume - than connect your AC adapter (USB cable connected to PC does not work), en Should start in bootloader menu
- If you can successfully start the bootloader menu, run the phone into recovery mode and than install the rom from external sd card
Una vez más, gracias

