There are various ways of resetting your router.  If you cannot access the web user interface, a 'reset' button is situated on the back of most devices.
Here are instructions to reset your device to default factory settings. Start with option 1, then 2 or 3 if not successful.

Option 1, "Soft Reset"

This will remove custom settings, but keep existing firmware

In the web user interface, there is an option in TOOL-> SYSTEM, to set the router back to factory default settings.

Option 2, "Hard Reset"

This will remove custom settings, but keep existing firmware

While power is ON, press and hold the 'reset' button on the back of your router for 10 seconds. Use a sharp object eg. a paperclip or a pen. Then release the button and wait until device restarts.

Option 3, "Crash Reset"

This will remove the current firmware. Next time you try to login, a message will show up, asking your for a firmware file/image. These can be downloaded from ftp.dlink.co.uk.

Power OFF the device. Press and hold the 'reset' button; while doing so, power on the device again and keep holding the 'reset' button for another 10 seconds. Then release the button and wait until device restarts.