Following methods tried in linksys wrt54g and tp link w741nd:

1. Hard Reset (“30/30/30 reset”) if configuration messed up
i. hold the reset button while power on or 30 seconds
ii. disconnect the power while continuing hold the reset button for 30 seconds
iii. while still holding it in, plug in the power again and keep holding the reset button for another 30 seconds
The router will back to default setting, i.e. 192.168.1.1

2. Firmware restore
My wrt54g got “no space left” so i decided to restore the firmware
i. telnet/ssh to the router and run the following commands to break the router, it will restore the router back to default setting:
mtd erase linux
reboot
ii. assign an static ip to your notebook e.g. 192.168.1.10
iii. open a command prompt and type: ping -t 192.168.1.1
iv. open another command prompt and type: tftp -i 192.168.1.1 put dd-wrt.xxxx.bin
do not press the enter
v. unplug and plug the router power supply
vi. as soon as the ping command got response again, press enter in the tftp windows

sample output:
C:\wrt>tftp -i 192.168.1.1 put dd-wrt.v24_mini_wrt54g.bin
Transfer successful: 3019776 bytes in 7 seconds, 431396 bytes/s

C:\wrt>tftp -i 192.168.1.1 put openwrt-wrt54g-squashfs.bin
Transfer successful: 2774016 bytes in 11 seconds, 252183 bytes/s

tips: find out the router mac address and add arp
arp -s 192.168.1.1 aa-bb-cc-dd-ee-ff

http://www.dd-wrt.com/wiki/index.php/Recover_from_a_Bad_Flash#Linksys_Firmware

http://www.dd-wrt.com/wiki/index.php/Reset_And_Reboot#Hard_Reset_.28.2230.2F30.2F30_reset.22.29

http://wiki.openwrt.org/doc/howto/generic.flashing.tftp?s[]=tftp