Hi all, I have an LG G3 that has started having the intermittent "no sim" errors similar to what has been reported here. When it first happened a reboot would not fix it and it got very bad over the next day or two. I got a new sim from AT&T and that fixed it completely for a while. Now it's back, and I've learned a trick that seems to work.
The problem for me at least seems to be related to the phone powering down unexpectedly. If I exhaust my battery and the phone shuts down quickly, then when I power back up it will be in the no sim state. I have found that if I power off, remove the sim, put the battery back in, power up with no sim at all, then power down, reinsert the sim, and power up, it clears the problem. Nothing else solves the problem for me - not reboots, and not airplane mode.
For me this definitely started happening right after my phone was force upgraded to marshmallow, and it seems like a software issue, not hardware.