Debricking
v7.2 (July 2012)
Disconnect the router from the internet, connect it with a PC via RJ45 cable and perform a 30/30/30 restart
Change the ip on the PC to 192.168.1.2, subnet mask must be 255.255.255.0 and restart the router by unplugging the power cable
Perform a failsafe boot, quickly use
telnet 192.168.1.1
If this didnt work check the ip adress on the PC or if you were too slow to connect during the failsafe mode. Still not working? Read this!
In telnet do the following things one after another:
Define a password
passwd
enter
yourpassword
enter
againyourpassword
enter
Kill the broken OWRT firmware
mtd -r erase rootfs_data
enter
firstboot
enter
reboot -f
After rebooting log in again just to check if the router is accessable. If you cant start again at the beginning.
Now change the network settings on the PC back to "Aquire IP adress automatically" and connect the router with the internet. Do a manual reboot by simply turning it off and back on again. The PC should now get an ip adress by DHCP. If this is the case you successfully resetted the router. If not repeat from the beginning or do this if you like.
Basically the router now works like after the initial flash to OWRT. Now you have to configure the internet connection and probably you want to install LUCI. After that open your browser, connect to 192.168.1.1 and log in with the data you entered via telnet.
v8.x note
Debricking see the above, but you will have to plug your cable to WAN port in failsafe to be able to telnet 192.168.1.1.
http://wiki.openwrt.org/toh/tp-link/tl-wr841nd 
