Essentially all broadband routers used for home networking provide a reset switch, often a very small, recessed button on the back of the unit. This button allows you to override the current state of the device and restore it to the default settings it had when it was first manufactured.
Some people don't realize that pressing a router's reset button for just a second or two may do nothing. Depending on the type of router and its current state (including the nature of any problems it may have), you may need to hold down the button longer. Networking enthusiasts have developed this so-called 30-30-30 hard reset procedure that should fully reset any home router to its default settings at any time. Follow these steps to perform a router hard reset:
1. when the unit is powered on, hold down its reset button for 30 seconds
2. while still holding down the reset button, unplug the router from power and hold for an additional 30 seconds
3. still holding down the reset button, turn on power to the router again and hold for 30 more seconds
After this process (a total of 90 seconds) is complete, your router should be restored to its factory default state. Note that your particular router may not require the full 30-30-30 procedure. For example, some routers can at times be hard reset after only 10 seconds and without power cycling. Nevertheless, I still recommended memorizing this 30-30-30 rule as a general guideline.