Short answer. An eBike that will not turn on is almost always a battery, key switch, main fuse, or display cable problem. Test each in order: charged battery, key switch continuity, main fuse in the battery housing, and the display cable connection.
Step by step
Confirm the battery is charged. Plug the charger in. If the charger light stays green immediately, the battery may already be fully drained past the recovery point or the BMS has shut off.
Check the key switch. On Rad, Aventon, and Super73 bikes, the battery has a key that must be turned to the on position. Wiggle and turn firmly. Corroded key switches are common.
Look for a blown main fuse. Most batteries have a small ATO or glass fuse near the discharge port. A blown fuse is a $2 fix but signals a short somewhere in the harness.
Reseat the display cable. The display cable is often the weakest link. Unplug, inspect the pins for corrosion or bent pins, and firmly reseat. A dead display can look like a dead bike.
Call a mechanic if none of the above. If the battery charges, the key works, the fuse is intact, and the display connector looks good, the fault is in the controller or main harness and needs proper diagnostic tools.