I (for unknown reasons) resolved to update the bios, what gave me a lot of problems when it bricked my laptop =/. So, after 4 days on research (and some hours  to do a backup, when no hope was left), I tried a last alternative, that you can follow below, with my commentaries in yellow:

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Replace the BIOS.WPH file with your notebook BIOS file, extracted from Lenovo BIOS update webpage. Rename your BIOS file to BIOS.WPH. -- I got v2.53

1) Create the CRISIS disk using USB flash drive
2) Make sure your notebook is power off -- I also removed the battery and the AC cable
3) Plug in the CRISIS disk to your Lenovo notebook
4) Press "Fn"+"R" key
5) While holding the "Fn"+"R" key -- I inserted the AC now -- , press power button
6) The notebook should power on and USB stick light showing activity
7) Stop pressing "Fn"+"R" key, after you see USB stick blinking LED
8) Your notebook should automatically restart with working BIOS in 2-3 minutes. -- For me, I needed to put the battery again

VERY IMPORTANT: Do not take off your battery!! It seems that before flashing, it verify AC/DC connection and battery level. If one of them is not ok, then reflash is  stopped. -- Yes, I know it now
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:D And now my lenovo g550 returns from the death \o/

Thanks for My Digital Life forum, and you can get Wincris from this link (it contains the lenovo v2.53 bios that I used, but I recommed you to get it from the site, extract and override mine)

Off course, I do not give you a warranty that this method will work for you... just worked for me, and I'm happy by not having to call the service

Good luck!