First of all, hi everybody! This is my first post on this forum, however I've been lurking around quite a bit: I hope this post will partially repay the community for all the help I got here.
Second, Many thanks to the developers of this wonderful firmware. I needed a universal repeater and bought the Wr842ND only once It got Openwrt support...
Third, sorry for my awful English... hope this post will make some sense...

Background:
A couple of days ago, I flashed a new OpenWRT build using 'sysupgrade -v' on a SSH shell USING A WIRELESS CONNECTION! 
Never do that!!! 
Long story short, the wireless connection went down in the middle of the upgrade and all I got was a rounded-edges, white brick.
I disconnected the power supply and reconnected after a bit: nothing happened, all the leds were off.
I had two choices: opening a warranty claim (I know it's unfair, it was my fault after all) or soldering a serial port (voiding the warranty) and restore from there. Moral issues apart, a claim would have costed me money and time while my soldering skills are less than optimal.
Never give up hope:
Today I tried this: remove power, press and keep pressed the reset button (on the back of the device) and connect the power again (reset button still pressed). About 10 seconds after giving power, I released the reset button. This time the USB led lit up. There was still life...
Next, I connected the router to my PC using an ethernet cable and the led of the connected eth port lit up too. Good. There was something passing over that eth port. A bit of wireshark sniffing and voila! I was able to reinstall the original tp-link firmware on the device.
Only for some minutes! Now I have opewrt again, of course smile
This is what I did, maybe It'll work with your little brick too:
- Power off the bricked device (disconnect the power plug)
- Press and keep pressed the reset button on the back
- Connect the power back (while keeping pressed the reset button)
- After 10 seconds, release the reset button
- USB led should be ON.
- Connect the WAN port of the router to your PC (Maybe you can use another LAN port, maybe not. Just use the WAN port and don't ask!)
- The WAN led should light up.
- Start Wireshark and sniff your PC's ethernet port. Try to locate packets coming from Atheros/xx:xx:xx, broadcasted, with protocol ARP.
   In my case, maybe it is the same for you (in this case you don't need wireshark at all), I saw messages like this: 
   "Who has 192.168.1.66? Tell 192.168.1.86" 
This means that my WR842ND had somehow got the IP 192.168.1.86 (don't ask me why) and was looking for a PC with address 192.168.1.66 (don't ask me why).
- So you have found one "Who has XXX.XXX.XXX.XXX?" message in your Wireshark log? Good. Keep note of the XXX.XXX.XXX.XXX address. Maybe it will be the same 192.168.1.66 as in my case, maybe not. Just write it down.
- In network control panel, assign to your PC the fixed IP: XXX.XXX.XXX.XXX  It's quite simple: the router's looking for XXX. Let's our PC be that XXX and check what the router wants to ask.
- If you are curious, check wireshark again: you'll see messages from the router (192.168.1.86 in my case) asking for the file "wr842ndv1_tp_recovery.bin". Protocol: TFTP. Perfect!!! The router's asking for a firmware!!!
- Install and set-up a TFTP server on your PC.
- Download the latest firmware for wr842nd from TP-Link.com. Extract the zip and RENAME the .BIN file inside to "wr842ndv1_tp_recovery.bin". Put the bin file in the folder served by your TFTP server.
- Disconnect the Ethernet cable from the PC (just to be sure). Start the TFTP server. Reconnect again the ethernet cable.  Your TFTP server, on your PC, should show the incoming connection (from the router) and the download (from PC to the router).
- Wait 10 minutes for the firmware to be processed/installed inside the router. After a while you should see it rebooting and all the leds come into life again.
- Remove the eth cable from the WAN port.
- Give your PC a dynamic address again (DHCP assigned)
- Connect the PC to one of the LAN ports. Go to: 192.168.0.1. Login with admin/admin
- Enjoy.

Nothing difficult. Only I wonder why the router got the address 192.168.1.86 and was looking for .66!!! I had never used the 192.168.1.0/24 class!
In my case, all went flawlessly. I think my bootloader was not gone for good, after all. I hope this will apply to you too in the unlucky event you brick your little box (which I hope not)!
Ask me if you need any more details.
Bye,
Isidoro