I have a LG G4. It's my daily driver.
First thing I would do is a battery pull. I would let the phone sit for 1/2 hour. I'd re-insert the battery and see if I could boot back up.
Second thing, I'd try booting into recovery. Power + Volume Down: Press both and hold. Does it boot into recovery?
Third thing, I would assume the battery is undercharged. I would use another microUSB cable, and charger, maybe even my PC USB port to try charging. Does the red-LED on the front come on? If so, that's a good sign. After charging for a bit, I'd try powering on.
Fourth thing, I'd try a new battery. Batteries die. It's rare to go from working properly to not working, I agree. If you have Amazon Prime, some deliveries are actually same day! Yeah, I'd throw $12-15 down on a replacement.