router hard reset

DO NOT attempt to reset or power cycle the router while it is still in the process of upgrading! Wait a couple of minutes after upgrade is complete and the router has rebooted itself.

Source
Hard Reset (aka 30/30/30 reset)
The following procedure will clear out the NVRAM and set dd-wrt back to default values:

• With the unit powered on, press and hold the reset button on back of unit for 30 seconds
• Without releasing the reset button, unplug the unit and hold reset for another 30 seconds
• Plug the unit back in STILL holding the reset button a final 30 seconds (please note that this step can put Asus devices into recovery mode...see note below!)

This procedure should be done BEFORE and AFTER every firmware upgrade/downgrade.
Do not use configuration restore if you change firmware builds (different svn build numbers).
[NOTE] - For Asus units, the standard 30/30/30 method will reset the router but you will need to use the Restore button followed by a Power cycle. The only exception (so far) is the RT-N16 model, follow the same method as stated above but use the WPS button instead of the Restore button. If all else fails to reset you can also do it by command line (all units). Login to the router via telnet/ssh and issue the command "erase nvram". This will reset the nvram partition to defaults for this Make of router.....redhawk

Erasing NVRAM:

As an alternative method, it is possible to restore DD-WRT to default settings without using the reset button (useful if you're locked out of the web interface or the reset button is disabled, broken, etc. or if your router just refuses to reset using 30/30/30).
[CAUTION] - On some devices, resetting NVRAM in this fashion may cause more problems than it solves. For example, Asus WL-500g and the Motorola WR850G bootloader will not recreate default values and will not boot properly after being reset. If you do this on a Siemens SE505 V1, your router will not be accessible to you anymore! You will have to reflash it with the stock firmware on IP address 192.168.1.1 (NOT 192.168.2.1 as the installation procedure says!!)
The following requires that DD-WRT already be installed on your router, and Telnet (or SSH) services be enabled and accessible.
Telnet (or SSH) into 192.168.1.1
Issue commands:

erase nvram
reboot

or

mtd -r erase nvram

The mtd method should be used with caution! The erase nvram;reboot method is much safer!
All should be in default settings now.

Default password

After resetting DD-WRT, you need to login with
the user name "root" 
and password "admin".