Short answer. An eBike throttle that stopped working is almost always a bad throttle unit, a disconnected throttle cable, or a controller fault. Replacement throttles are cheap and swap in under 10 minutes.
Step by step
Confirm the throttle is enabled. Some eBikes have a settings option to disable throttle in Class 1 mode. Check the display settings first.
Reseat the throttle cable. Follow the cable from the throttle to the harness junction and reseat the connector. Look for corroded pins.
Test with a known good throttle. If a spare throttle works, the original is dead. If neither works, the controller or harness is the problem.
Check for error codes. E02 on Rad and 022 on Bafang both point to throttle faults. See the specific brand error code guides.