I upgrade the firmware of my transmitter by mistake. (the old transmitter) and after that, is not working anymore.

any idea how can i get it back to work?

Can you be more specific, do you mean the RC as it has the transmitter with it, try to Rebind it, there is a process in the manual
Check back and let us know!

Check here for helpful information you can provide for best diagnosis
http://forum.dji.com/forum.php?m ... ight=Please&lang=en

I had this happen to me, i kept trying to upgrade it until it turned on, and it worked.