On the back of the router there should be a hole that's pretty small. You'll need a pin or tiny screwdriver to insert into said hole to push down a button at the end. (yes, the absurd innuendo potential in that sentence is not lost on me)

Keep said button pressed for a minute.

This will hard reset the router back to it's default settings.

Alternatively you can just unplug it and leave it for around 5 minutes, be sure to turn all the connected devices off too, then gradually turn everything back on one by one. This will perform a soft reset, if that doesn't work then the hard reset is your best bet.