I'm sorry to hear about the issue you're experiencing with your Royal Enfield Thunderbird. If the gear lever has come off while the motorcycle is still in gear, you can try the following steps to start the bike and shift to neutral:
Ensure the motorcycle is on a flat surface and in an area free from traffic.
Make sure the engine kill switch is in the "ON" position.
Depress the clutch lever fully with your left hand and keep it held in.
Use your right foot to carefully push the gear lever up repeatedly while applying gentle pressure.
Gradually release the clutch lever while still trying to shift the gear lever upward.
If successful, the motorcycle should shift to neutral and the engine will be disengaged from the transmission.
Once the bike is in neutral, you can start the engine by turning the ignition key or pressing the electric start button, depending on the model.
If you're unable to shift to neutral using the above steps, it would be best to seek assistance from a qualified mechanic or Royal Enfield service center. They will have the expertise and tools necessary to diagnose and resolve the issue with the gear lever.
Remember to prioritize your safety while dealing with any motorcycle issue. If you're unsure or uncomfortable with the situation, it's always best to contact professional help or towing services to avoid any further damage or accidents.