Routers can and do go wrong. Fortunately, their settings are held in non-volatile memory, so switching them off and back on again usually fixes most common faults.

If this doesn't work, it may be because you have changed the configuration. In this case you should reset the router to its factory defaults and start again. With some devices you can do this from the management interface, using a browser. In others there will be a hardware reset button. Check the manual for details.

Routers do an awful lot of processing work and can get quite warm, so make sure they've got adequate ventilation and that any cooling grilles have not been covered up.