What happens if the turbo is not working?

Could you elaborate on the potential implications if a turbocharger is malfunctioning? I'm particularly interested in understanding the effects on engine performance, fuel efficiency, and any potential safety hazards that may arise. Additionally, I'd like to know if there are any specific warning signs or symptoms that drivers should be aware of to detect a turbocharger issue in its early stages. Lastly, how critical is it to have a professional inspect and repair a non-working turbocharger promptly?