When a Fire Stick won't connect to Wi-Fi, it's usually because of a signal strength issue. The situation isn't always simple though, and you'll find that issues with your router, the Fire Stick itself ...
Well, before going to some hard troubleshooting guide, rule out the possibility of hardware built up by restarting the Firestick. To be more cautious, remove the Firestick from the TV, and Monitor, ...