I just tried to revert to original firmware from Chaos Calmer and bricked my router by the look of it.
I did read about the "boot" images to avoid and used a stripped file.
But instead of using the "sysupgrade" command, I used the Web GUI. Is this what I did wrong? Or maybe I should have waited for more than two minutes?
I've followed these instructions but I couldn't connect to the router with telnet. I'm not even sure if I can enter failsafe mode.
I know this is a serious problem because OpenWRT was mostly overwritten, but I'm asking before I throw my only OpenWrt-capable router in the garbage.
Thank you for your attention.

(Last edited by Slackware on 8 Sep 2015, 21:35)

Your router is recoverable with the TFTP server method since your bootloader is unharmed

Solved!
I had tried the TFTP method, but your answer made me try again more carefully, and it worked this time.
Thank you.

Details:
I pressed the reset button for 5 seconds after powering on the router.
I used Tftpd server. It was silent all the time. Only the LEDs signaled something different was going on.
The firmware file was named "mr3420v2_tp_recovery.bin", as instructed on the wiki.
I started the server after setting the static IP configuration.
In the server GUI window, I selected "192.168.0.66" as "Server interfaces".