If your wifi isn't working after a thunderstorm, it's likely because of storm-related damage to your router or modem. Lightning can cause power surges that fry electronic equipment, and thunderstorms can also bring down trees and power lines, disrupting your home's internet connection.
If you suspect that storm damage is the cause of your wifi woes, the first thing to do is check for any physical damage to your router or modem. If you see any burned out components or charring, it's likely that lightning was the culprit. You'll need to replace your damaged equipment in order to get back online.
Even if there's no visible damage, it's still possible that a power surge from the lightning strike has caused internal damage to your equipment. In this case, you may be able to revive your equipment by resetting it according to the manufacturer's instructions. If that doesn't work, unfortunately you'll need new gear.
Once you've replaced any damaged equipment, take steps to protect your new investment from future storms. Unplugging electronics during a thunderstorm is always a good idea, but if you can't do that (or if you forget), consider investing in a surge protector for peace