Thanks Alan for you answer.
The reverse gear only worked when I turned the handlebar direction, so it seemed that there were some bad contact (wires cut or something), and I think that a resistance of the controller could has been burned to squeeze the throttle to the maximum and suddenly workk again due to when I turned the handlebar and making contact with the accelerator to the maximum has been able to burn something inside the controller.
I opened the controller and it seems that there is something a little bit burned, I scraped it a bit and I have put it all back together, and now it does not cut, but it has lost almost all the power, both forward and reverse. So, I believe and I hope that replacing the controller with a new one solves everything. Some recommendations from where to buy it, taking into account that I live in Spain, to avoid extra customs and other expenses.
Thank you very much,
Rafa