Summarize the following:
If you can keep a calm demeanor, you'll help the autistic person feel calmer too.  Keep a patient and understanding attitude. Show them the same kindness that you'd want other people to show you if you were at your wits' end. Never shout at, scold, or punish an autistic person for being upset. They aren't doing this on purpose, and being unkind will only make it worse. If you can't control yourself, it's better to leave than to make the situation worse. Sometimes, they may be overwhelmed, and need quiet time. Other times, they might be experiencing difficult emotions related to something in their life (like a bad grade at school or an argument with a friend).  During severe sensory overload, people who are ordinarily verbal may suddenly lose the ability to speak. This is due to severe overstimulation, and will pass with relaxation time. If someone has lost the ability to speak, ask only yes/no questions that they can answer with thumbs up/thumbs down. If you cannot, encourage any people in the room to leave. Explain that unexpected noise and movement are hard for the autistic person right now, and she would be happy to hang out again sometime later. Sometimes, the person might want you there to keep them company and help them calm down. Other times, they might want to be alone for a while. Either way, don't take it personally.   If they can't speak right now, let them answer with a thumbs up/thumbs down. Or you can say "Do you want me to stay or leave?" and point at the ground and at the door, and then let them point to where they want you to be. If a small child wants to be left alone, you can sit across the room and do something quiet (like playing on your phone or reading a book) so there is still an adult present. When they're distressed, they may be unable to think clearly, and they may have trouble doing simple tasks like taking off an uncomfortable sweater or getting a drink of water. Help them out, without infringing on their personal space.   If they're tugging at uncomfortable clothing, offer to help them remove it. (Don't try to remove clothes without permission, as this can be startling and upsetting.) If they're trying to drink from the sink, get a cup for them. Move dangerous or breakable objects out of their way. Put a pillow or folded-up jacket under their head to protect it, or put their head on your lap if it's safe.   If they are throwing things, it might be that the throwing motion calms them down. Try giving them something that can be thrown safely (like a throw pillow). Let them throw it, and then retrieve it so they can throw it again. This can calm them. If you don't feel safe getting close to them, then don't. Let them continue until they calm down and wear themselves out. Parents, teachers, and caregivers may know how to help. They may be able to offer specific insight about the autistic person's particular needs.  Police are not usually trained to help with autistic meltdowns, and they may worsen the situation or hurt your autistic loved one. Instead, get someone who the autistic person knows and trusts.

Summary:
Take a moment to calm yourself. Ask what's wrong, if the person is able to speak. Take them to a quiet place. Ask if they want you to stay with them. Help them with any difficult tasks. Keep them safe if they are thrashing, flailing, or throwing things. Get help if you don't know what to do.