My Power button broke on my G5, I'm pretty sure it happened after I banged it against the wall. What can I do to fix it?
Read this: thedroidguy.com/2016/09/fix-lg-g5-wont-turn-troubleshooting-guide-1060702
If nothing helps take the phone apart (go to youtube for guides). See if the button is stuck. If nothing seems to be broken buy a cheap button replacement from china (ebay is your friend). Again check youtube for a guide on how to connect the new button to the motherboard.
If you're not confortable taking your device apart take it to a service. Guessing you don't have warranty anymore.