Check to see if the wheels in the back have been damaged. In the mayhem of rock climbing, wheels can get broken and therefore become unresponsive. If this is the case, check out the Wheel Replacement Guide.
If the wheels are fine, it could be a few other electronics issues. First, try replacing the batteries. There is only one motor in the back so the two in the front could be taking the last remaining power. If that doesn't work, there could be a problem with the rear axle. If so, check out the Axle Replacement Guide. If that still doesn't solve your problem, there is probably a wiring issue. In such a case, try replacing the circuit board. Check out the Circuit Board Replacement Guide for help.