Unlikely a schematic exists. There are some partial schematics for assorted controllers over the years posted around the forums, drawn up by users troubleshooting various problems, but they only apply to the specific version of the specific model of that brand of controller. Others may be very similar, but not necessarily identical, possibly still useful for basic testing of some hardware.
However, almost all of the functionality of a controller is within the software in the MCU, and that varies widely, and is pretty much a black box. Only the few open source projects have code available to the public.
As a side note, the battery meter on the display is normally fed from the display's own MCU, not from the controller. If it is a multiple-voltage system, it may be autodetecting the wrong battery type, one of higher voltage than yours actually is, and so thinks yours is empty.
So, some troubleshooting is likely to be needed, so some questions:
Did it ever work correctly?
If so, what happened between the time it worked and the time it didn't?
What voltage is the battery? (a 36v battery will look empty to a 48v controller, and will probalby be low enough even at full charge to engage the controller LVC)
What do you mean by "register brake levers"?
By "not go at all", what input control method are you using? PAS? Throttle? Walk mode (if any)?
What assist mode/level is set in the display, if it uses modes or levels?
What does the manual (if any) say that level requires / does?
