That is illegal and could land you into far more serious trouble than you probably realise. Note that this is not a guide to hack into Wi-Fi networks. Before you head down that path, here are a few tips to recover your saved Wi-Fi passwords.
Until you buy a new phone, or a guest wants to access the network on their smartphone - that's when a frantic search for the Wi-Fi password ensues, before giving up and resetting your wireless router. Most people configure Wi-Fi networks once, enter the password on all their devices and forget about it completely. People keep forgetting their passwords all the time, but if there's one kind of password that no one bothers to remember it has to be the Wi-Fi password.