Not sure on Rad, but some bikes won't power on if things are not right. there is a controller between the LCD and the motor. Did you remove the battery, and replace it back in the mount? When you go to power it on, watch if you see any flicker on the display. Hopefully some Rad users will stop by shortly, and tell you were to check, not sure if there are any internal connectors on the Rad, but I thought not. Check the cables one more time, look for connectors all the way between the display and the rear wheel. Many people don't find the loose connector until the 2nd or 3rd time.
Hard to really work, but you can quickly try turning it on with the steering turned all the way right and then all the way left just incase it triggers because of a broken wire in one of the cables or a loose connector near the steering.
Contact Rad if you have not already; they might have some quick tips, but I presume you have already done this though.