If the battery is say 2 years old, try another as old batteries can create all sorts of troubles. Since the SIM card isn't being read I'd also open it up, remove the SIM, memory card, battery and get the canned air out to give the slots a blast. Wipe the contacts with a clean clean (gently.)
I've lost count of old battery problems. It's a cheap part and worth trying. Some folk disagree and if so my nod is to a new phone after one last thing. Factory reset and don't install apps. Test as-is for a few days.
I own a LG G3 and lately it has been behaving quite od. Sometimes it closes and activate it self manny times in a row. Power shuts of and on.
Sometimes it doesn't read the Sim card although it's always installed. So i can't make phonecalls or send text and people can't reach me. The battery gets empty very fast during the day. Part of the answer might be that the battery might be getting old but what about the rest of the phone's behavior?
What can I do in order to solve these problems?